Do not block nightly builds on tier 2 platforms.

Author: gebner

.github/workflows/nightly-build.yml

@@ -29,6 +29,7 @@ jobs:
   publish:
     runs-on: ubuntu-latest
     needs: build-all
+    if: always()
     steps:
       - uses: actions/checkout@master
         with:
@@ -64,6 +65,9 @@ jobs:
           # ^ Download all artifacts into the same dir.
           # Each of them is a single file, so no clashes happen.
 
+      - name: Check that tier 1 platform builds succeeded
+        run: test -f artifacts/fstar-Linux-x86_64.tar.gz
+
       # For debugging.
       - run: |
           echo LOCAL