Add pub get step.

Hoping to solve this error in workflow environment:

Unhandled exception:
Invalid argument (uri): Unknown package: Instance of '_SimpleUri'
#0      _resolveUri.<anonymous closure> (package:dartdoc/src/generator/resource_loader.dart:34:9)
#1      _RootZone.runUnary (dart:async/zone.dart:1612:54)
#2      _FutureListener.handleValue (dart:async/future_impl.dart:152:18)
#3      Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:704:45)
#4      Future._propagateToListeners (dart:async/future_impl.dart:733:32)
#5      Future._completeWithValue (dart:async/future_impl.dart:539:5)
#6      Future._asyncCompleteWithValue.<anonymous closure> (dart:async/future_impl.dart:577:7)
#7      _microtaskLoop (dart:async/schedule_microtask.dart:40:21)
#8      _startMicrotaskLoop (dart:async/schedule_microtask.dart:49:5)
#9      _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:120:13)
#10     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:185:5)
Error: Process completed with exit code 255.

.github/workflows/docs.yml

@@ -13,6 +13,9 @@ jobs:
         with:
           channel: stable
 
+      - name: Get Flutter Packages
+        run: flutter pub get
+
       - name: Build Documentation
         run: |
           dartdoc