We should replace it with whatever the user set, not start again.

When given $PATH as part of a search path, we guess again instead of
substituting what the user already set.

refdb: disable concurrent compress in the threading tests on Windows

Various fixes

This is far from an ideal situation, but this causes issues on Windows which
make it harder to develop anything, as these tests hit issues which relate
specifically to the Windows filesystem like permission errors for files we
should be able to access. There is an issue likely related to the ordering of
the repack, but there's enough noise that it does not currently help us to run
this aspect of the test in CI.