Replace assignTestFlightGroup boolean with testFlightGroup choice

Lets the dispatcher pick which TestFlight group an iOS build gets
assigned to (QA Team, Software Mansion, or none). Previously the
input was a boolean that always pointed at "QA Team". Nightly build
keeps current behavior by passing "QA Team" explicitly.

.github/workflows/nightly-build.yml

@@ -77,7 +77,7 @@ jobs:
     uses: ./.github/workflows/build-submit-ios.yml
     with:
       profile: testflight
-      assignTestFlightGroup: true
+      testFlightGroup: "QA Team"
     secrets: inherit
 
   android: