CI: add hlint workflow

Author: Anton-Latukha

.github/workflows/hlint.yml

@@ -0,0 +1,17 @@
+name: "Checking with HLint"
+
+on:
+  pull_request:
+    branches:
+      - '**'
+
+jobs:
+  build10:
+    name: "HLint check"
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: "HLint via ./fmt.sh"
+      run: |
+        ./fmt.sh

.github/workflows/test.yml

@@ -154,11 +154,7 @@ jobs:
                 ${{ env.cache-name }}-${{ runner.os }}-${{ matrix.ghc }}-
                 ${{ env.cache-name }}-${{ runner.os }}-
 
-      - run: cabal update
-
-      - name: "HLint via ./fmt.sh"
-        run: |
-          ./fmt.sh
+      - run: cabal v2-update
 
       # repeating builds to workaround segfaults in windows and ghc-8.8.4
       - name: Build