Actions: allow manual triggering

I tried to get Actions to run on contributor's PRs back in:
    2023-09-19 Actions: don't ignore if '/' in branch name
    748cb93
but it didn't work.

After reading the docs some more, my theory is that the since the
previous yaml ran "on a push to any branch", that meant "on a push to
any branch in the Tarsnap/tarsnap repository", which didn't apply to
third parties, since they were pushing to their own fork.

Adding "workflow_dispatch" should allow us to manualy trigger a check
for third-party PRs.

.github/workflows/compile.yml

@@ -1,11 +1,13 @@
 name: Compile & test
 
-# Run whenever we push to any branch other than [coverity-scan].
+# Run whenever we push to any branch other than [coverity-scan], or if we
+# manually trigger a workflow.
 on:
   push:
     branches:
     - '**'
     - '!coverity-scan'
+  workflow_dispatch:
 
 env:
   # Shared variables amongst all projects / platforms / compilers.