Linux customer_testing: "Flutter users should use `flutter pub` instead of `dart pub`."

Start with dev/customer_testing/ci.sh and reproduce the Linux customer_testing shard from the linked build output. Trace the dependency-resolution command that produces the dart pub error, then rerun the shard to confirm the customer-testing dependency setup succeeds and the intermittent failure is resolved.

example: https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20customer_testing/101590/overview

▌11:04:37▐ STARTING ANALYSIS
▌11:04:37▐ SHARD=customer_testing
▌11:04:37▐ Running customer testing
RUNNING: cd .; git fetch origin master
workingDirectory: /b/s/w/ir/x/w/flutter, executable: git, arguments: [fetch, origin, master]
From https://github.com/flutter/flutter
 * branch                    master     -> FETCH_HEAD
 * [new branch]              master     -> origin/master
ELAPSED TIME: 0.347s for git fetch origin master in /b/s/w/ir/x/w/flutter
RUNNING: cd .; git branch -f master origin/master
workingDirectory: /b/s/w/ir/x/w/flutter, executable: git, arguments: [branch, -f, master, origin/master]
branch 'master' set up to track 'origin/master'.
ELAPSED TIME: 0.043s for git branch -f master origin/master in /b/s/w/ir/x/w/flutter
RUNNING: cd .; git checkout 2b6a349952afbd20535325a4bd029786b365ae0c
workingDirectory: /b/s/w/ir/x/w/flutter, executable: git, arguments: [checkout, 2b6a349952afbd20535325a4bd029786b365ae0c]
HEAD is now at 2b6a349952a Merge branch 'master' into flakey-web-tool
ELAPSED TIME: 0.587s for git checkout 2b6a349952afbd20535325a4bd029786b365ae0c in /b/s/w/ir/x/w/flutter
RUNNING: cd dev/customer_testing; ../../ci.sh 
workingDirectory: /b/s/w/ir/x/w/flutter/dev/customer_testing, executable: ./ci.sh, arguments: []
Resolving dependencies in `/b/s/w/ir/x/w/flutter`...
Because flutter_api_samples requires the Flutter SDK, version solving failed.

Flutter users should use `flutter pub` instead of `dart pub`.
╔═╡ERROR #1╞════════════════════════════════════════════════════════════════════
║ Command: ../../ci.sh 
║ Command exited with exit code 1 but expected zero exit code.
║ Working directory: /b/s/w/ir/x/w/flutter/dev/customer_testing
║ stdout and stderr output:
║ Resolving dependencies in `/b/s/w/ir/x/w/flutter`...
║ Because flutter_api_samples requires the Flutter SDK, version solving failed.
║ 
║ Flutter users should use `flutter pub` instead of `dart pub`.
╚═══════════════════════════════════════════════════════════════════════════════
▌11:04:39▐ Test failed.

10/25 of the last runs have failed