Replaced environ.get with os.getenv for Windows compatibility

[BenMcLean]

No description provided.

[neikeq]

Is there a problem with environ.get on Windows?

Also there's one more usage of environ.get:

godot-mono-builds/build_mono_android.py

Line 301 in cce3073

ccache_path = environ.get('CCACHE', '')

[akien-mga]

Is there a problem with environ.get on Windows?

Probably not, but if HOME is undefined, then environ.get("HOME") will be None which breaks the follow-up code. While getenv("HOME", "") gives a default fallback value if the env variable is undefined.

This PR needs to be rebased and squashed into one commit.

[neikeq]

I wasn't thinking properly when I first commented. The scripts don't support Windows (unless it's cygwin or WSL).