actions/checkout: always checkout pinned commits

With the latest performance improvements, this only adds up to 1s, but
reduces complexity in managing pinned checkouts explicitly, especially
when we need *both* pinned checkouts in the next commit.

(cherry picked from commit c18d1e4af8)

.github/workflows/check.yml

@@ -103,7 +103,6 @@ jobs:
         uses: ./.github/actions/checkout
         with:
           merged-as-untrusted-at: ${{ inputs.mergedSha }}
-          pinned-from: trusted
           target-as-trusted-at: ${{ inputs.targetSha }}
 
       - uses: cachix/install-nix-action@fc6e360bedc9ee72d75e701397f0bb30dce77568 # v31
@@ -117,7 +116,7 @@ jobs:
           pushFilter: -source$
 
       - name: Build codeowners validator
-        run: nix-build nixpkgs/trusted/ci --arg nixpkgs ./nixpkgs/pinned -A codeownersValidator
+        run: nix-build nixpkgs/trusted/ci --arg nixpkgs ./nixpkgs/trusted-pinned -A codeownersValidator
 
       - uses: actions/create-github-app-token@0f859bf9e69e887678d5bbfbee594437cb440ffe # v2.1.0
         if: github.event_name == 'pull_request_target' && vars.OWNER_RO_APP_ID