Add manual trigger to e2e-test github action

Add a manual trigger for running the end-to-end tests with github actions.
The manual trigger takes as input a signed tag for determining which
artifacts to fetch from the build server.

.github/workflows/desktop-e2e.yml

@@ -1,6 +1,11 @@
 ---
 name: Desktop - End-to-end testing on supported platforms
 on:
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: 'Signed tag'
+        required: true
   schedule:
     - cron: '0 0 * * *'
   # TODO(markus,david): Remove before merging to main
@@ -25,6 +30,7 @@ jobs:
         shell: bash -ieo pipefail {0}
         env:
           TEST_OSES: ${{ matrix.os }}
+          TAG: ${{ github.event.inputs.tag }}
         run: |
           ./test/ci-runtests.sh ${{ matrix.os }}