🔒️(ci) pin every action to a full commit hash

A tag and a branch both move, so actions/checkout@v6 and
numerique-gouv/action-trivy-cache@main ran whatever the owner had last
pushed. Each of the 65 uses now names a 40-character commit, with the
version it resolved to in a trailing comment.

dependabot.yml keeps the hash and that comment moving together.

.github/dependabot.yml

@@ -0,0 +1,8 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "⬆️(ci)"