Auto merge of #362 - larsbergstrom:windows_caching, r=aneeshusa
Auto merge of #362 - larsbergstrom:windows_caching, r=aneeshusa

Add caching of Cargo and Rust bits to Windows

r? @edunham @aneeshusa

Fixes #358

I decided to use the msys home dir instead of `%APPDATA%` for consistency with the other unix builds, but could be convinced otherwise if we get to a point where Servo is not building inside of the msys/mingw environment.

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setupminimal/saltfs,setupminimal/saltfs,ALikhachev/saltfs,servo/saltfs,UK992/saltfs,ALikhachev/saltfs,rwaweber/saltfs,UK992/saltfs,rwaweber/saltfs,servo/saltfs,ALikhachev/saltfs,setupminimal/saltfs,UK992/saltfs,rwaweber/saltfs