fix: Codecov upload fails on Dependabot PRs (missing token)

[moliholy]

Summary

Codecov coverage upload fails on Dependabot PRs because the branch is treated as protected and no CODECOV_TOKEN is provided.

Example

Failed run on dependabot/cargo/keccak-0.1.6

error - Commit creating failed: {"message":"Token required because branch is protected"}
Codecov: Failed to properly create commit

Cause

codecov/codecov-action@v4 requires a CODECOV_TOKEN for protected/non-default branches. Dependabot PRs run with a read-only GITHUB_TOKEN and don't have access to it. Combined with fail_ci_if_error: true, this blocks the entire CI job.

Options

1. Add CODECOV_TOKEN as a repository secret and pass it to the action
2. Set fail_ci_if_error: false so coverage upload failures don't block CI
3. Both

[moliholy]

cc @al3mart @AlexD10S