[9.20] fix: ci: Fix .respdiff-recent-named anchor to work when the ABI changes

[9.20] fix: ci: 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.

This also is not a problem on non-MR pipelines: the latest released version
was used as a reference there, so the .so versions would differ.

See the 9.20 pre-backport branch and the jobs:

- Broken: https://gitlab.isc.org/isc-projects/bind9/-/jobs/6951217
- Fixed: https://gitlab.isc.org/isc-projects/bind9/-/jobs/6951220

Backport of MR !11616

Merge branch 'backport-stepan/respdiff-fails-on-abi-breakage-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11619