🧺 make TmpCwd more fault tolerant

TmpCwd used to raise FileNotFound exception when the processes cwd is
pointing to a non-existent directory.  This change makes it so that
TmpCwd doesn't raise this exception anymore.
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f