ci: Fix docs workflow (#921)

Author: mtrezza

.github/workflows/release-manual-docs.yml

@@ -23,6 +23,11 @@ jobs:
  with:
  channel: 'stable'
  cache: true
+ - name: Install dart dependencies
+ run: dart pub get --directory 'packages/dart'
+ - name: Install flutter dependencies
+ if: env.package == 'flutter'
+ run: dart pub get --directory 'packages/flutter'
  - name: Generate Docs
  run: dart doc ./packages/${{ env.package }}/ -o ./.api_docs/${{ env.package }}/
  - name: Deploy