Run pre-commit hooks in dynamic pipeline

Use gitlab dynamic child pipelines feature to have one source of truth
for the pre-commit jobs, the pre-commit config file.

Use one cache per pre-commit. This should reduce the "fetching cache"
time steps in gitlab-ci, since each job will have a separate cache with
only its hook installed.

.gitlab-ci/pre-commit-dynamic-stub.yml

@@ -0,0 +1,17 @@
+---
+# stub pipeline for dynamic generation
+pre-commit:
+  tags:
+  - light
+  image: 'ghcr.io/pre-commit-ci/runner-image@sha256:aaf2c7b38b22286f2d381c11673bec571c28f61dd086d11b43a1c9444a813cef'
+  variables:
+    PRE_COMMIT_HOME: /pre-commit-cache
+  script:
+  - pre-commit run -a $HOOK_ID
+  cache:
+    key: pre-commit-$HOOK_ID
+    paths:
+    - /pre-commit-cache
+  parallel:
+    matrix:
+    - HOOK_ID: