* 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
* build: add config to build Electron for tests
* build: run nightly linux builds for the 3-0-x branch
* build: gn: update CI config for linux builds
- run GN debug and testing builds for PRs (as FYI)
- do not run tests for GN debug builds
- run GN release builds nightly
* build: gn: run testing builds on CI instead of release on Windows
Should save some time since release builds set 'official_build=true'
which make the builds take much more time.
* build: gn: use testing config to run CI jobs on Mac
Tests should run faster for testing builds.
