Try installing GHCJS via HVR's PPA

.github/workflows/main.yml

@@ -118,3 +118,16 @@ jobs:
     - name: Test
       run: |
         stack test --system-ghc
+
+  ghcjs:
+    name: stack / ghcjs 8.4
+    runs-on: "ubuntu-18.04"
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Install ghcjs
+      run: |
+        sudo add-apt-repository ppa:hvr/ghcjs
+        sudo apt-get update -y
+        sudo apt-get install ghcjs-8.4