* build: add GIT_CACHE_PATH
The merge at https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1105473 was to specify git cache entirely from the environment variable [and] removal of the general --cache-dir option from gclient
* docs: document GIT_CACHE_PATH in GN build
* fix: try to fix appveyor-gn env syntax
Devs can still locally override or add settings by adding lines after
the `import` statement in args.gn.
This will help make sure that everyone has a common set of build args,
and will allow us to update everyone's build args in git.
