fix(@angular/build): watchers outside the workspace via parcel didn't work

The `dirKey` entry in `this.pendingSubscriptions` caused the following `if` to never be true, as `this.isCoveredByExistingExternal(dirKey)` is always true as long as `dirKey` is in there. But at this point it isn't pending anymore, as we just `await`ed it the LoC prior.

This bug caused the `else` to always run, which did `sub.unsubscribe()`, which killed the just spawned watcher. The effect was that a directory outside the workspace wasn't watched anymore.

E.g. this structure:

```
apps/my-angular-app
libs/my-angular-sdk
node_modules/@my/sdk => symlink to libs/my-angular-sdk
```

When you ran `ng serve` in `apps/my-angular-app` (which has imports to `@my/sdk`) and made a change in `libs/my-angular-sdk`, then the watcher didn't rebuild (which it did before in 21.x).

(cherry picked from commit 83eedfa)

Author: sod

packages/angular/build/src/utils/watcher.ts

@@ -530,6 +530,8 @@ class ParcelExternalManager {
 
     try {
       const sub = await subPromise;
+      this.pendingSubscriptions.delete(dirKey);
+
       if (this.externalDirFiles.has(dirKey) && !this.isCoveredByExistingExternal(dirKey)) {
         this.extraSubscriptions.set(dirKey, sub);