Make SpecBuildInterface pickleable (#25628)

* Add a __reduce__ method to SpecBuildInterface

This class was confusing pickle when being serialized,
due to its scary nature of being an object that disguise
as another type.

* Add more MacOS tests, switch them to clingo

* Fix condition syntax

* Remove Python v3.6 and v3.9 with macOS

.github/workflows/unit_tests.yaml

@@ -337,10 +337,14 @@ jobs:
       run: |
         brew install dash fish gcc gnupg2 kcov
     - name: Run unit tests
+      env:
+        SPACK_TEST_SOLVER: clingo
       run: |
         git --version
         . .github/workflows/setup_git.sh
         . share/spack/setup-env.sh
+        $(which spack) bootstrap untrust spack-install
+        $(which spack) solve zlib
         if [ "${{ needs.changes.outputs.with_coverage }}" == "true" ]
         then
           coverage run $(which spack) unit-test -x