Explicitly exit the process to not wait for hanging promises

As with other actions like setup-node, I'm seeing 2-4 minute delays in post cache actions lately. Apparently this is because of a change in node behavior: ruby/setup-ruby#543 (comment)

The fix, as with other actions, is to explicitly exit so as not to wait for hanging promises.

src/setup-pdm.ts

@@ -67,6 +67,9 @@ async function run(): Promise<void> {
   catch (error: any) {
     core.setFailed(error.message)
   }
+  // Explicit process.exit() to not wait for hanging promises,
+  // see https://github.com/actions/setup-node/issues/878
+  process.exit()
 }
 
 run()