Windows builder is not caching cargo and rust deps

[larsbergstrom]

Need to set the CARGO_HOME and SERVO_CACHE_DIR environment variables to something sensible to avoid wasting a ton of time on our Windows builder.

Changes should be made to this file:
https://github.com/servo/saltfs/blob/master/buildbot/master/files/config/environments.py

[aneeshusa]

Specifically, these variables should be added to the build_windows expression.

[jeandudey]

What about using %APPDATA%\.cargo for CARGO_HOME and %APPDATA%\.servo for SERVO_CACHE_DIR?

[larsbergstrom]

@jeandudey That sounds great to me, and much better than my other hacky ideas! The one good thing to test locally is to make sure that env vars get properly expanded when set and used by Cargo, as otherwise the files will still end up in the directory that gets cleared every time!