refdb: disable concurrent compress in the threading tests on Windows
refdb: disable concurrent compress in the threading tests on Windows

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.

stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2