Manual override environment variable

[smashwilson]

Support an environment variable that lets users override the bundled binaries with an external git binary that they've built elsewhere. This provides advanced users an escape hatch to use on older Linux distributions than we support, as opposed to them just being SOL.

We'd have to be careful to specify the git version we expect -- part of the benefit of using git-kitchen-sink is not having to worry about compatibility with older git versions, after all. Maybe include the git version in the environment variable name?

As suggested in desktop/dugite-native#13 (comment).

[shiftkey]

Prior art: we already set a TEST_WITH_LOCAL_GIT flag when executing tests which change the behaviour of resolving the Git directory.

https://github.com/desktop/git-kitchen-sink/blob/master/lib/git-process.ts#L76-L82

We could make this environment variable more generic, e.g. USE_ALTERNATE_GIT=/some/path, and trust this value instead of doing the platform-specific lookup we're currently doing. This would mean we'd trust the local version of Git to be configured right, and not set GIT_TEMPLATE_DIR and PREFIX on our end.