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

lib/spack/spack/spec.py

@@ -1019,7 +1019,9 @@ class SpecBuildInterface(lang.ObjectWrapper):
 
     def __init__(self, spec, name, query_parameters):
         super(SpecBuildInterface, self).__init__(spec)
-
+        # Adding new attributes goes after super() call since the ObjectWrapper
+        # resets __dict__ to behave like the passed object
+        self.token = spec, name, query_parameters
         is_virtual = spack.repo.path.is_virtual(name)
         self.last_query = QueryState(
             name=name,
@@ -1027,6 +1029,9 @@ def __init__(self, spec, name, query_parameters):
             isvirtual=is_virtual
         )
 
+    def __reduce__(self):
+        return SpecBuildInterface, self.token
+
 
 @lang.lazy_lexicographic_ordering(set_hash=False)
 class Spec(object):