Add explicit process.exit() to avoid wait for hanging promises

When using the `@actions/cache` library to save cache entries, it seems that one
or more Promises remain unresolved after the save completes.
With Node20 this causes a delay when exiting the process: the default behaviour
now wait for these Promises to complete. Adding an explicit `Process.exit()`
removes the delay, returning to the Node 16 behaviour.

Fixes #1038

src/main.ts

@@ -29,6 +29,9 @@ export async function run(): Promise<void> {
             core.info(error.stack)
         }
     }
+
+    // Explicit process.exit() to prevent waiting for hanging promises.
+    process.exit()
 }
 
 run()

src/post.ts

@@ -16,11 +16,13 @@ export async function run(): Promise<void> {
     } catch (error) {
         if (error instanceof PostActionJobFailure) {
             core.setFailed(String(error))
-            return
+        } else {
+            handleFailure(error)
         }
-
-        handleFailure(error)
     }
+
+    // Explicit process.exit() to prevent waiting for promises left hanging by `@actions/cache` on save.
+    process.exit()
 }
 
 function handleFailure(error: unknown): void {