ci(installer): split installer step per shell (shell: can't be an expression)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0139nGyeFNACmifj6pRLSVRs

Author: JohnMcLear

.github/workflows/installer-test.yml

@@ -131,8 +131,20 @@ jobs:
       - name: Pre-install pnpm
         run: npm install -g pnpm
 
-      - name: Run bin/installer.ps1 against this commit
-        shell: ${{ matrix.shell }}
+      # `shell:` can't take an expression, so one step per shell.
+      - name: Run bin/installer.ps1 against this commit (PowerShell 7)
+        if: matrix.shell == 'pwsh'
+        shell: pwsh
+        env:
+          ETHERPAD_DIR: ${{ runner.temp }}\etherpad-installer-test
+          ETHERPAD_REPO: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.repo.clone_url || format('{0}/{1}.git', github.server_url, github.repository) }}
+          ETHERPAD_BRANCH: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.ref || github.ref_name }}
+          NO_COLOR: "1"
+        run: ./bin/installer.ps1
+
+      - name: Run bin/installer.ps1 against this commit (Windows PowerShell 5.1)
+        if: matrix.shell == 'powershell'
+        shell: powershell
         env:
           ETHERPAD_DIR: ${{ runner.temp }}\etherpad-installer-test
           ETHERPAD_REPO: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.repo.clone_url || format('{0}/{1}.git', github.server_url, github.repository) }}