Add a GitHub workflow to manually test the detached mode

To better assess how useful the detached mode is, and whether it handles
all of the corner cases correctly (client connects only during the post
job phase, job is canceled while waiting for the session to end, etc),
let's have a GitHub workflow that can be triggered manually.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.github/workflows/manual-detached-test.yml

@@ -0,0 +1,20 @@
+name: Test detached mode
+on: workflow_dispatch
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: ./
+        with:
+          limit-access-to-actor: true
+          detached: true
+      - run: |
+          echo "A busy loop"
+          for value in $(seq 10)
+          do
+            echo "Value: $value"
+            echo "value $value" >>counter.txt
+            sleep 1
+          done