CI: Use absolute path to cargo on macos-13-large

Also pins the macos version so we don't get silently switched to
aarch64 which is tested separately.

.github/workflows/build-and-test-x86.yml

@@ -78,8 +78,8 @@ jobs:
           name: meson-test-logs-${{ matrix.target }}-${{ matrix.build.name }}
           path: |
              ${{ github.workspace }}/build/meson-logs/testlog-*.txt
-  test-on-macos-latest:
-    runs-on: macos-latest
+  test-on-macos-13:
+    runs-on: macos-13 # select an x86_64 image
     steps:
       - name: install prerequisites
         env: