spec: ensure_valid_variants() should not validate concrete specs

Variants in concrete specs are "always" correct -- or at least we assume
them to be b/c they were concretized before. Their variants need not match
the current version of the package.

lib/spack/spack/spec.py

@@ -3052,6 +3052,10 @@ def ensure_valid_variants(spec):
         Raises:
             spack.variant.UnknownVariantError: on the first unknown variant found
         """
+        # concrete variants are always valid
+        if spec.concrete:
+            return
+
         pkg_cls = spec.package_class
         pkg_variants = pkg_cls.variants
         # reserved names are variants that may be set on any package