workflows: Only run `cmake --install` on Unix platforms

The syntax used in this step assumes a Unix shell.

Signed-off-by: Simon McVittie <smcv@collabora.com>

.github/workflows/main.yml

@@ -86,6 +86,7 @@ jobs:
     - name: Build
       run: cmake --build build/ --config Release
     - name: Install
+      if: matrix.platform.shell == 'sh'
       run: |
         set -eu
         rm -fr DESTDIR-cmake