feat: add ability to override cache location

[ckerr]

I'm writing tests in another repo that inject fake downloads into the process and don't want them to pollute the real cache on my system. Also, I don't want the real cache's checksums to cause my tests to fail because they don't match the fakes' checksums.

The workflow I have in mind is that the test setup code will create a tmpdir and the set ELECTRON_GET_CACHE_DIR to point at that tmpdir.

[shenzhuxi]

https://www.electron.build/multi-platform-build.html The document also needs a update. "ELECTRON_CACHE" has been replaced by "electron_config_cache", and it's also case sensitive.

[erickzhao]

@shenzhuxi this would need to be raised with the maintainers of https://github.com/electron-userland/electron-builder.