update git workflow to use the main branch of the SDK (#3251)

Author: jakemac53

.github/workflows/dart.yml

@@ -16,7 +16,7 @@ jobs:
       - uses: actions/checkout@8e5e7e5ab8b370d6c329ec480221332ada57f0ab
       - uses: dart-lang/setup-dart@d6a63dab3335f427404425de0fbfed4686d93c4f
         with:
-            sdk: dev
+            sdk: main
       - name: dart pub get (specification)
         run: dart pub get
         working-directory: specification
@@ -37,7 +37,7 @@ jobs:
       - uses: actions/checkout@8e5e7e5ab8b370d6c329ec480221332ada57f0ab
       - uses: dart-lang/setup-dart@d6a63dab3335f427404425de0fbfed4686d93c4f
         with:
-            sdk: dev
+            sdk: main
       - name: dart pub get (accepted/future-releases/0546-patterns/exhaustiveness_prototype)
         run: dart pub get
         working-directory: accepted/future-releases/0546-patterns/exhaustiveness_prototype