Fix .respdiff-recent-named anchor to work when the ABI changes

Fix .respdiff-recent-named anchor to work when the ABI changes

Previously, on 9.20 and 9.18, both builds (reference and the version
being tested) would use the same .so files which lead to a crash if the
ABI changed.

Use `git worktree` to get completely separate build environment for the
reference version.

This is not a problem on 9.21 as Meson is smart and covers this mistake,
but apply the fix to it as well for consistency.

(cherry picked from commit a719341314d5fdc0a7b61082aea2fac6db26a8b6)