workflows/{check-shell,manual-nixos,manual-nixpkgs}: use get-mege-commit action

This makes a difference for the case of a merge conflict: In that case,
the magic `.../merge` branch actually points to the *last test merge
commit* that was successful, which might not contain the latest head
commit in any way. Running the tests on that commit is heavily
misleading. By using the get-merge-commit action, we run on the PR's
head commit in this case, which is much better.

(cherry picked from commit 039a954892)

.github/workflows/check-shell.yml

@@ -32,7 +32,14 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: refs/pull/${{ github.event.pull_request.number }}/merge
+          sparse-checkout: .github/actions
+      - name: Check if the PR can be merged and get the test merge commit
+        uses: ./.github/actions/get-merge-commit
+        id: get-merge-commit
+
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          ref: ${{ steps.get-merge-commit.outputs.mergedSha }}
 
       - uses: cachix/install-nix-action@526118121621777ccd86f79b04685a9319637641 # v31
 

.github/workflows/manual-nixos-v2.yml

@@ -34,7 +34,14 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: refs/pull/${{ github.event.pull_request.number }}/merge
+          sparse-checkout: .github/actions
+      - name: Check if the PR can be merged and get the test merge commit
+        uses: ./.github/actions/get-merge-commit
+        id: get-merge-commit
+
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          ref: ${{ steps.get-merge-commit.outputs.mergedSha }}
 
       - uses: cachix/install-nix-action@526118121621777ccd86f79b04685a9319637641 # v31
         with:

.github/workflows/manual-nixpkgs-v2.yml

@@ -21,7 +21,14 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
-          ref: refs/pull/${{ github.event.pull_request.number }}/merge
+          sparse-checkout: .github/actions
+      - name: Check if the PR can be merged and get the test merge commit
+        uses: ./.github/actions/get-merge-commit
+        id: get-merge-commit
+
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          ref: ${{ steps.get-merge-commit.outputs.mergedSha }}
 
       - uses: cachix/install-nix-action@526118121621777ccd86f79b04685a9319637641 # v31
         with: