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

dist/main/index.js

@@ -140762,6 +140762,7 @@ function run() {
                 core.info(error.stack);
             }
         }
+        process.exit();
     });
 }
 exports.run = run;

dist/post/index.js

@@ -138072,10 +138072,12 @@ function run() {
         catch (error) {
             if (error instanceof errors_1.PostActionJobFailure) {
                 core.setFailed(String(error));
-                return;
             }
-            handleFailure(error);
+            else {
+                handleFailure(error);
+            }
         }
+        process.exit();
     });
 }
 exports.run = run;