Add way to skip desktop build/test for downstream tests (#34322)

* Add way to skip desktop build/test for downstream tests

* Iterate

.github/workflows/build-and-test.yaml

@@ -26,7 +26,13 @@ on:
                 type: boolean
                 required: false
                 default: false
-                description: "A boolean to skip the playwright check itself while still creating the passing check. Useful when only running in Merge Queues."
+                description: "A boolean to skip everything while still creating the passing check. Useful when only running in Merge Queues."
+
+            skip-desktop:
+                type: boolean
+                required: false
+                default: false
+                description: "A boolean to skip building & testing Element Desktop."
 
             matrix-js-sdk-sha:
                 type: string
@@ -237,7 +243,7 @@ jobs:
         name: "Prepare Element Desktop"
         uses: ./.github/workflows/build_desktop_prepare.yaml
         needs: prepare
-        if: needs.prepare.outputs.skip == 'false'
+        if: needs.prepare.outputs.skip == 'false' && inputs.skip-desktop != true
         permissions:
             contents: read
         with: