diag: trigger on this branch, since workflow_dispatch needs the default branch

A workflow_dispatch-only workflow is not dispatchable until the file exists on
the default branch. Putting a throwaway diagnostic on main to be able to run it
is the opposite of what it is for, so it triggers on pushes to its own branch.

Author: myobie

.github/workflows/diag-pipe-holder.yml

@@ -4,8 +4,12 @@ name: Diag pipe holder
 # conformance run finishes but its output pipe does not close, WHICH process is
 # holding the write end?
 #
-# It is manual only. Nothing triggers it but a person.
+# It runs only on its own throwaway branch. A workflow_dispatch alone is not
+# dispatchable until the file exists on the default branch, which is exactly
+# what this must not do.
 on:
+  push:
+    branches: [diag/pipe-holder]
   workflow_dispatch:
 
 jobs: