Use xargs instead of parallel

Parallel was hanging for some reason

.github/workflows/main.yml

@@ -39,4 +39,8 @@ jobs:
       # Get the names of the packages being added to the archive, and feed these
       # to opam-ci-check
       - name: Lint newly archived packages
-        run: git diff --name-only origin/main packages | sed 's:.*/\(.*\)/opam:\1:' | parallel opam exec -- opam-ci-check lint -r . --checks=archive-repo
+        run: |
+          git diff --name-only origin/main packages \
+          | sed 's:.*/\(.*\)/opam:\1:' \
+          | xargs --max-lines=1 --max-procs=4 opam exec -- opam-ci-check lint -r . --checks=archive-repo \
+          | grep "Error in"