glib: Use g_getenv everywhere instead of getenv

Merged Nirbheek Chauhan requested to merge nirbheek/glib:g-getenv-everywhere into master

getenv() doesn't work well on Windows, f.ex., it can't fetch env vars set with SetEnvironmentVariable(). This also means that it doesn't work at all when targeting UWP since that's the only way to set env vars in that case.

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