Adjust CI env variables as workaround for sbt scala binary version resolution bug

Author: prolativ

.github/workflows/ci.yaml

@@ -89,6 +89,8 @@ jobs:
            github.event_name == 'workflow_dispatch'
            && github.repository == 'lampepfl/dotty'
          )"
+    env:
+      RELEASEBUILD: yes
 
     steps:
       - name: Set JDK 16 as default
@@ -384,6 +386,9 @@ jobs:
            && github.repository == 'lampepfl/dotty'
          )"
 
+    env:
+      RELEASEBUILD: yes
+
     steps:
       - name: Set JDK 8 as default
         run: echo "/usr/lib/jvm/java-8-openjdk-amd64/bin" >> $GITHUB_PATH