Try gcc 9

Author: cknitt

.github/workflows/ci.yml

@@ -22,6 +22,11 @@ jobs:
         with:
           repository: rescript-lang/rescript-compiler
 
+      - name: Install compiler
+        uses: rlalik/setup-cpp-compiler@master
+        with:
+          compiler: gcc-9
+
       - run: sudo apt install -y --no-install-recommends opam musl-tools
 
       - run: opam --version