System-specific environment variables on Windows pollute remote cache

Description of the problem / feature request / question:

When building C++ rules on Windows with MSVC, system-specific environment variables are part of the inputs, i.e.

SET TEMP=C:\Users\<my_user>\AppData\Local\Temp
SET TMP=C:\Users\<my_user>\AppData\Local\Temp
SET PATH="...;C:\Program Files (x86)\Skype\Phone\;C:\Program Files (x86)\Bitvise SSH Client;..."

For remote caching, this prevents sharing outputs between different systems.

Environment info

• Operating System: Windows 10

• Bazel version (output of bazel info release): release 0.5.4rc1

Have you found anything relevant by searching the web?

https://groups.google.com/forum/#!topic/bazel-discuss/4GV8lJzfeGQ