fail ci with codecov error + workflow_dispatch

- `fail_ci_if_error: true` => if the codecov upload errors, CI fails (probably due to secrets / codecov token)
- manually triggering CI is enabled with `workflow_dispatch`

.github/workflows/CI.yml

@@ -4,6 +4,7 @@ on:
     branches: [master]
     tags: ['*']
   pull_request:
+  workflow_dispatch:
 jobs:
   test:
     name: Julia ${{ matrix.version }} - ${{ matrix.os }} ${{ matrix.arch }}
@@ -41,6 +42,7 @@ jobs:
         with:
           files: lcov.info
           token: ${{ secrets.CODECOV_TOKEN }}
+          fail_ci_if_error: true
   docs:
     name: Documentation
     runs-on: ubuntu-latest