Test bootstrapping in a workflow (#25138)

Add a workflow to test bootstrapping clingo on 
different platforms so that we can detect changes 
that break it.

Compute `site_packages_dir` in `bootstrap.py` as it was
before #24095, until we figure a better way to override
that attribute.

var/spack/repos/builtin/packages/python/package.py

@@ -920,8 +920,12 @@ def site_packages_dir(self):
         try:
             return self.get_python_lib(prefix='')
         except (ProcessError, RuntimeError):
-            return os.path.join(
-                'lib', 'python{0}'.format(self.version.up_to(2)), 'site-packages')
+            return self.default_site_packages_dir
+
+    @property
+    def default_site_packages_dir(self):
+        python_dir = 'python{0}'.format(self.version.up_to(2))
+        return os.path.join('lib', python_dir, 'site-packages')
 
     @property
     def easy_install_file(self):