CI: reference actions by full GitHub URL

.github/workflows/ci.yml

@@ -20,11 +20,13 @@ jobs:
       # and drops a subpath such as /git (actions/checkout#1370, #1242), so it
       # cannot clone from imsc.uni-graz.at/git. ischanx/checkout keeps the path.
       # Pinned to the commit of its v6.0.2 tag; its v4 tags lack the fix.
-      - uses: ischanx/checkout@b3a7a36c84c480d7e1a6f9f7ca7bb55dc519f0ba # v6.0.2
-      - uses: pnpm/action-setup@v4
+      # Actions use full GitHub URLs: short names resolve against the
+      # instance's default actions registry, not GitHub.
+      - uses: https://github.com/ischanx/checkout@b3a7a36c84c480d7e1a6f9f7ca7bb55dc519f0ba # v6.0.2
+      - uses: https://github.com/pnpm/action-setup@v4
         with:
           version: 11
-      - uses: actions/setup-node@v4
+      - uses: https://github.com/actions/setup-node@v4
         with:
           node-version: ${{ matrix.node }}
           cache: pnpm