find: add --hashes shortcut for piping to other commands (#38663)

People frequently ask us how to pipe `spack find` output to other commands, and we tell
them to do things like this:

```console
$ spack find --format "/{hash}" | spack uninstall -ay
```

Sometimes users don't know about hash references and come up with potentially ambiguous
formulations like this:

```console
spack find --format {name}@{version}%{compiler} | spack uninstall -ay
```

Since this is a common enough thing to want to do, and to make it more obvious how, this
PR adds a `-H` / `--hashes` as a shortcut, so you can now just do:

```console
spack find -H | spack uninstall -ay
```

lib/spack/spack/cmd/find.py

@@ -30,6 +30,14 @@ def setup_parser(subparser):
         default=None,
         help="output specs with the specified format string",
     )
+    format_group.add_argument(
+        "-H",
+        "--hashes",
+        action="store_const",
+        dest="format",
+        const="{/hash}",
+        help="same as '--format {/hash}'; use with xargs or $()",
+    )
     format_group.add_argument(
         "--json",
         action="store_true",