security: pin GitHub Actions to commit SHAs, add Dependabot

Pin all 22 action references across ci.yml, publish.yml, and
attest-release.yml to immutable commit SHAs. Mutable tags can be
force-pushed by attackers (cf. TeamPCP supply chain campaign).

Add Dependabot for github-actions to get weekly PRs when pinned
actions have new versions.

.github/workflows/ci.yml

@@ -10,8 +10,8 @@ jobs:
   python:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5  # v4
+      - uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065  # v5
         with:
           python-version: "3.12"
       - name: Install dependencies
@@ -22,8 +22,8 @@ jobs:
   javascript:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-node@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5  # v4
+      - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020  # v4
         with:
           node-version: 20
       - name: Install and build