--source: the tmpbin python -> python2 link breaks in several setups

The bug is in build_node_from_src in nodeenv.py, which looks up python2 and links it as tmpbin/python. Start by running the issue's reproduction script against that function, once per case, to see each failure. Done means the five cases (python2.7 only, dangling link, stale link, pyenv-style shim, relative PATH entry) either build or fail with a clear message, and the fix settles whether the link approach should change.

On Python 3, build_node_from_src looks up python2 and points python at it through a link in tmpbin (master, ecb4b74):

        python2_path = shutil.which('python2')
        if not python2_path:
            raise OSError(
                'Python >=3.0 virtualenv detected, but no python2 '
                'command (required for building node.js) was found'
            )
        logger.debug(' * Temporarily pointing python to %s', python2_path)
        node_tmpbin_dir = join(src_dir, 'tmpbin')
        node_tmpbin_link = join(node_tmpbin_dir, 'python')
        mkdir(node_tmpbin_dir)
        if not os.path.exists(node_tmpbin_link):
            callit(['ln', '-s', python2_path, node_tmpbin_link])
        env['PATH'] = '{}:{}'.format(node_tmpbin_dir,
                                     os.environ.get('PATH', ''))

PR #429 keeps this code for node.js before 13 (the lookup) and before 8.8 (the link). It breaks in these cases:

1. Only python2.7 is on PATH. A Python 2.7 installed with make altinstall has python2.7 but no python2. nodeenv raises the "no python2 command" error, although node's own ./configure wrapper (6.12+ / 8.5+) tries python2.7 first.
2. A dangling tmpbin/python from an earlier run. src is kept after a failed build or with --no-clean-src, and --force reuses it. os.path.exists follows the link, so once the old python2 is gone, ln -s fails with File exists.
3. A stale tmpbin/python from an earlier run. If the old target still exists, the build uses it instead of the python2 found now.
4. python2 is a pyenv shim. pyenv shims pick the program by their own name (program="${0##*/}", then pyenv exec "$program"), so a link named python to ~/.pyenv/shims/python2 runs pyenv exec python. With e.g. pyenv global 3.12.0 2.7.18 that is Python 3.12.
5. A relative PATH entry. shutil.which returns a relative path (bin/python2), and ln -s stores it relative to tmpbin/, so the link dangles and ./configure can't find python.

The link is also made by running ln -s through callit, while the rest of the module uses os.symlink.

Reproduce

On master (ecb4b74) on macOS, where /usr/bin has no python2, with fake scripts in place of node's configure, make and python2. Run from a nodeenv checkout:

T=/tmp/y; mkdir -p $T
git show ecb4b74:nodeenv.py > $T/nodeenv.py
PY=$(command -v python3)
mkscript() { mkdir -p "$(dirname "$1")"; printf '#!/bin/sh\n%s\n' "$2" > "$1"; chmod +x "$1"; }
mkscript $T/tools/make 'exit 0'
mkscript $T/py27/python2.7 'echo "python2.7 runs"'
mkscript $T/py2old/python2 'echo "OLD python2 runs"'
mkscript $T/py2new/python2 'echo "NEW python2 runs"'
mkscript $T/shims/python2 'echo "shim dispatches to: ${0##*/}"'
# every case gets a node source dir whose configure runs `python`
newcase() { mkscript $T/$1/src/node-src/configure 'exec python'; }
cat > $T/run.py <<'EOF'
import sys, types
sys.path.insert(0, '/tmp/y')
import nodeenv
work = sys.argv[1]
args = types.SimpleNamespace(load_average=None, jobs='2', without_ssl=False,
    debug=False, profile=False, make_path='/tmp/y/tools/make', verbose=True)
try:
    nodeenv.build_node_from_src(work + '/env', work + '/src', work + '/src/node-src', args)
except OSError as e:
    print('OSError:', e)
EOF

1. Only python2.7:

newcase c1
env PATH="$T/py27:/usr/bin:/bin" $PY $T/run.py $T/c1

OSError: Python >=3.0 virtualenv detected, but no python2 command (required for building node.js) was found

2. Dangling link:

newcase c2; mkdir -p $T/c2/src/tmpbin; ln -s $T/gone/python2 $T/c2/src/tmpbin/python
env PATH="$T/py2new:/usr/bin:/bin" $PY $T/run.py $T/c2

ln: /tmp/y/c2/src/tmpbin/python: File exists
OSError: Command ln -s /tmp/y/py2new/python2 /tmp/y/c2/src/tmpbin/python failed with error code 1

3. Stale link:

newcase c3; mkdir -p $T/c3/src/tmpbin; ln -s $T/py2old/python2 $T/c3/src/tmpbin/python
env PATH="$T/py2new:/usr/bin:/bin" $PY $T/run.py $T/c3

OLD python2 runs

4. A shim that dispatches on its name like pyenv's:

newcase c4
env PATH="$T/shims:/usr/bin:/bin" $PY $T/run.py $T/c4

shim dispatches to: python

5. Relative PATH entry:

newcase c5; mkscript $T/c5/bin/python2 'echo "python2 runs"'
(cd $T/c5 && env PATH="bin:/usr/bin:/bin" $PY $T/run.py $T/c5)
readlink $T/c5/src/tmpbin/python

./configure: line 2: exec: python: not found
OSError: Command ./configure --prefix=/tmp/y/c5/env failed with error code 127
bin/python2