Destroy runner on allocation stopped

Destroying the runner when Nomad informs us about its allocation being stopped, fixes the error of executions running into their timeout even if the allocation was stopped long ago.

internal/runner/runner.go

@@ -59,7 +59,8 @@ type Runner interface {
 	GetFileContent(path string, content http.ResponseWriter, privilegedExecution bool, ctx context.Context) error
 
 	// Destroy destroys the Runner in Nomad.
-	Destroy() error
+	// Iff local is true, the destruction will not be propagated to external systems.
+	Destroy(local bool) error
 }
 
 // NewContext creates a context containing a runner.