Create coverage directory beforehand

Author: karoliineh

.github/workflows/coverage.yml

@@ -61,7 +61,9 @@ jobs:
       - name: Test
         env:
           BISECT_FILE: ${{ github.workspace }}/_build/coverage/bisect
-        run: opam exec -- dune runtest --force --instrument-with bisect_ppx
+        run: |
+          mkdir -p _build/coverage
+          opam exec -- dune runtest --force --instrument-with bisect_ppx
 
       - run: opam exec -- bisect-ppx-report send-to Coveralls
         env: