CI: remove codecov on main merge (starkware-libs#1480)

* CI: remove codecov on main merge

Currently, codecov reports are not always in adequation with the
code being pushed in a PR.
Disabling on main merge, ensure that maintainers can evaluate
a PR to be mergeable, even if the codecov report is not valid.

* fix: conditioned only the codecov step and not the whole workflow

* fix: make the restriction on main ref instead of event_name

.github/workflows/ci.yml

@@ -20,6 +20,7 @@ jobs:
       - uses: Swatinem/rust-cache@v2
       - run: cargo llvm-cov nextest --all-features --lcov --output-path lcov.info
       - uses: codecov/codecov-action@v3
+        if: github.ref != 'refs/heads/main'
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
           files: lcov.info