Make source directories ephemeral when running scripts

Make source directories ephemeral when running scripts

Various tools like to write to the source directory, which we want
to avoid. Let's make source directories ephemeral when running scripts
so tools can write to it as much as they want but we can throw away all
those changes when we're done running scripts.

Specifically, this makes running rpmbuild as documented in
docs/building-rpm-from-source.md a lot nicer as the source directory won't be
polluted with all manner of temporary files anymore.