Run ghcjs CI on ubuntu 20.04

Author: maksbotan

.github/workflows/ghcjs.yml

@@ -8,8 +8,8 @@ on:
 
 jobs:
   ghcjs:
-    name: ubuntu-18.04 / ghcjs 8.4
-    runs-on: "ubuntu-18.04"
+    name: ubuntu-20.04 / ghcjs 8.4
+    runs-on: "ubuntu-20.04"
 
     steps:
     - uses: actions/checkout@v2