Pass _all_ changed files to Ormolu

Previously, only the last file ended up getting formatted, because the
filenames ended up joined on a single line.

Author: sellout

.github/workflows/ci.yaml

@@ -48,7 +48,7 @@ jobs:
           files: |
             **/*.hs
             **/*.hs-boot
-          separator: "\n"
+          separator: "\n  "
       - uses: haskell-actions/run-ormolu@15b0083a0ef416915994fb511652b187f6026a40 # v15
         with:
           version: ${{ env.ormolu_version }}