Packaging: fix status badges in frozen readme

GitHub status badge urls no longer allow
branch=TAG. Must use branch=BRANCH or
tag=TAG. (This was only visible in
published description on PyPI.)

(Actions job filter still requires branch=TAG_OR_BRANCH)

hatch_build.py

@@ -12,10 +12,16 @@ def freeze_readme_versions(text: str, version: str) -> str:
     (This assumes version X.Y will be tagged "vX.Y" in git.)
     """
     release_tag = f"v{version}"
-    return re.sub(
     # (?<=...) is "positive lookbehind": must be there, but won't get replaced
-        # GitHub Actions build status: branch=main --> branch=vX.Y.Z:
-        r"(?<=branch[=:])main"
+    text = re.sub(
+        # GitHub Actions badge: badge.svg?branch=main --> badge.svg?tag=vX.Y.Z:
+        r"(?<=badge\.svg\?)branch=main",
+        f"tag={release_tag}",
+        text,
+    )
+    return re.sub(
+        # GitHub Actions status links: branch:main --> branch:vX.Y.Z:
+        r"(?<=branch:)main"
         # ReadTheDocs links: /stable --> /vX.Y.Z:
         r"|(?<=/)stable"
         # ReadTheDocs badge: version=stable --> version=vX.Y.Z: