ci(workflow): add workflow_dispatch trigger

Allows manual triggering to test adaptive concurrency control.

Author: AlexMikhalev

.github/workflows/vm-execution-tests.yml

@@ -20,6 +20,7 @@ on:
       - 'scratchpad/firecracker-rust/**'
       - 'scripts/test-vm-execution.sh'
       - '.github/workflows/vm-execution-tests.yml'
+  workflow_dispatch:  # Allow manual triggering
 
 env:
   CARGO_TERM_COLOR: always