Make clingo the default solver (#25502)

Modifications:
- [x] Change `defaults/config.yaml`
- [x] Add a fix for bootstrapping patchelf from sources if `compilers.yaml` is empty
- [x] Make `SPACK_TEST_SOLVER=clingo` the default for unit-tests
- [x] Fix package failures in the e4s pipeline

Caveats:
1. CentOS 6 still uses the original concretizer as it can't connect to the buildcache due to issues with `ssl` (bootstrapping from sources requires a C++14 capable compiler)
1. I had to update the image tag for GitlabCI in e699f14.  
1. libtool v2.4.2 has been deprecated and other packages received some update

.github/workflows/unit_tests.yaml

@@ -225,23 +225,25 @@ jobs:
       if: ${{ needs.changes.outputs.with_coverage == 'true' }}
       env:
           HOME: /home/spack-test
+          SPACK_TEST_SOLVER: original
       run: |
           whoami && echo $HOME && cd $HOME
           git clone https://github.com/spack/spack.git && cd spack
           git fetch origin ${{ github.ref }}:test-branch
           git checkout test-branch
-          share/spack/qa/run-unit-tests
+          bin/spack unit-test -x
     - name: Run unit tests (only package tests)
       if: ${{ needs.changes.outputs.with_coverage == 'false' }}
       env:
           HOME: /home/spack-test
           ONLY_PACKAGES: true
+          SPACK_TEST_SOLVER: original
       run: |
           whoami && echo $HOME && cd $HOME
           git clone https://github.com/spack/spack.git && cd spack
           git fetch origin ${{ github.ref }}:test-branch
           git checkout test-branch
-          share/spack/qa/run-unit-tests
+          bin/spack unit-test -x -m "not maybeslow" -k "package_sanity"
 
   # Test RHEL8 UBI with platform Python. This job is run
   # only on PRs modifying core Spack
@@ -268,6 +270,7 @@ jobs:
       shell: runuser -u spack-test -- bash {0}
       run: |
           source share/spack/setup-env.sh
+          spack -d solve zlib
           spack unit-test -k 'not cvs and not svn and not hg' -x --verbose
   # Test for the clingo based solver (using clingo-cffi)
   clingo-cffi: