'spack spec --yaml' command now writes build deps to yaml (#12317)

This allows "spack spec --yaml" to generate a spec YAML file that can
be used with "spack install -f". Before, this would fail in cases
where the spec had build dependencies.

lib/spack/spack/cmd/spec.py

@@ -14,6 +14,7 @@
 import spack.cmd
 import spack.cmd.common.arguments as arguments
 import spack.spec
+import spack.hash_types as ht
 
 description = "show what would be installed, given a spec"
 section = "build"
@@ -63,7 +64,7 @@ def spec(parser, args):
                 spec.concretize()
 
             # use write because to_yaml already has a newline.
-            sys.stdout.write(spec.to_yaml())
+            sys.stdout.write(spec.to_yaml(hash=ht.build_hash))
             continue
 
         kwargs['hashes'] = False  # Always False for input spec