ci: Add Stackage LTS 18 to Stack test matrix

.github/workflows/stack.yml

@@ -31,6 +31,9 @@ jobs:
           - resolver: lts-17
             ghc: 8.10.4
             yaml: stack-master.yaml
+          - resolver: lts-18
+            ghc: 8.10.7
+            yaml: stack-master.yaml
 
     steps:
       - name: Clone project
@@ -55,7 +58,8 @@ jobs:
             #
 
       - name: Install GHC
-        # use system ghc in stack, don't waste GH Actions cache space
+        # use system ghc (if available) in stack, don't waste GH Actions cache space
+        continue-on-error: true
         run: |
           set -ex
           sudo apt install -y ghc-${{ matrix.ghc }}