dependabot: update only the GitHub Actions

.github/dependabot.yml

@@ -1,6 +1,7 @@
-# Dependabot version updates.
-# The hash-locked environments in requirements/*.lock are pinned evidence for the published
-# run and the frozen pipeline, so they are updated by hand, never automatically.
+# Dependabot version updates, for the GitHub Actions only.
+# The Python dependencies are pinned by hash in requirements/preparation.lock, which is evidence for
+# the published run, so they are updated by hand, never automatically. A pip update here would only
+# change preparation.in or pyproject.toml and leave the lock behind (as in #232 and #233).
 version: 2
 updates:
   - package-ecosystem: "github-actions"
@@ -17,19 +18,3 @@ updates:
       prefix: "chore(deps)"
       include: "scope"
 
-  - package-ecosystem: "pip"
-    directory: "/"
-    schedule:
-      interval: "weekly"
-      day: "monday"
-      time: "09:00"
-    open-pull-requests-limit: 5
-    labels:
-      - "dependencies"
-      - "python"
-    commit-message:
-      prefix: "chore(deps)"
-      include: "scope"
-    ignore:
-      # Bit-exact recomputation of the released run depends on the exact PyTorch build.
-      - dependency-name: "torch"