bugfix: sort variants in spack info --variants-by-name (#41389)

This was missed while backporting the new `spack info` command from #40326.

Variants should be sorted by name when invoking `spack info --variants-by-name`.

lib/spack/spack/cmd/info.py

@@ -327,7 +327,7 @@ def _variants_by_name_when(pkg):
     """Adaptor to get variants keyed by { name: { when: { [Variant...] } }."""
     # TODO: replace with pkg.variants_by_name(when=True) when unified directive dicts are merged.
     variants = {}
-    for name, (variant, whens) in pkg.variants.items():
+    for name, (variant, whens) in sorted(pkg.variants.items()):
         for when in whens:
             variants.setdefault(name, {}).setdefault(when, []).append(variant)
     return variants