[9.18] 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-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11620

.gitlab-ci.yml

@@ -2053,16 +2053,15 @@ respdiff-third-party:
       artifacts: true
   script:
     - cd ${CI_PROJECT_DIR}
-    - mkdir version-under-test
-    - cp -ar bin version-under-test/
     - BASELINE=${CI_MERGE_REQUEST_TARGET_BRANCH_NAME:-$BIND_BASELINE_VERSION}
     - git fetch --unshallow origin ${BASELINE}
-    - git checkout FETCH_HEAD
+    - git worktree add baseline-build FETCH_HEAD
+    - cd baseline-build
     - autoreconf -fi
     - *configure
     - make -j${BUILD_PARALLEL_JOBS:-1} V=1
     - cd "$CI_PROJECT_DIR"/bind9-qa/respdiff
-    - bash respdiff.sh -s named -q "${PWD}/100k_mixed.txt" -c 3 -w "${PWD}/rspworkdir" "${CI_PROJECT_DIR}/version-under-test" "${CI_PROJECT_DIR}/bin/named/named"
+    - bash respdiff.sh -s named -q "${PWD}/100k_mixed.txt" -c 3 -w "${PWD}/rspworkdir" "${CI_PROJECT_DIR}" "${CI_PROJECT_DIR}/baseline-build/bin/named/named"
 
 respdiff:recent-named:
   <<: *respdiff_recent_named