Fix send-email action version in nightly workflow (#527)

- Added the missing version info: `@master` for send-email action.

Author: lahirumaramba

.github/workflows/nightly.yml

@@ -54,7 +54,7 @@ jobs:
 
     - name: Send email on failure
       if: failure()
-      uses: firebase/firebase-admin-node/.github/actions/send-email
+      uses: firebase/firebase-admin-node/.github/actions/send-email@master
       with:
         api-key: ${{ secrets.OSS_BOT_MAILGUN_KEY }}
         domain: ${{ secrets.OSS_BOT_MAILGUN_DOMAIN }}
@@ -69,7 +69,7 @@ jobs:
 
     - name: Send email on cancelled
       if: cancelled()
-      uses: firebase/firebase-admin-node/.github/actions/send-email
+      uses: firebase/firebase-admin-node/.github/actions/send-email@master
       with:
         api-key: ${{ secrets.OSS_BOT_MAILGUN_KEY }}
         domain: ${{ secrets.OSS_BOT_MAILGUN_DOMAIN }}