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\\AppData\Local\Temp
SET TMP=C:\Users\\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

The issue mentions Windows 10, MSVC C++ rules, and remote caching but names no source file or test. Start by reproducing a remote-cache build with differing TEMP, TMP, and PATH values; done means system-specific environment values no longer prevent sharing valid outputs between systems.

Written by the indexing model from the issue text.