fix(ci): stop upgrading npm in publish workflow

[bntvllnt]

Summary

• remove the npm install -g npm@latest step from the publish workflow
• use the npm bundled with actions/setup-node@v4 instead of mutating the runner's global npm install
• unblock the canary/release publish jobs after the post-merge Publish workflow for PR chore: prep v0.2.0 release and upgrade Convex #40 failed in the npm upgrade step

Validation

• inspected the failed run: Publish Canary failed before publish with MODULE_NOT_FOUND: promise-retry while running npm install -g npm@latest
• verified the workflow diff is limited to removing the failing upgrade step

Failure fixed

• failed run: https://github.com/vllnt/convex-api-keys/actions/runs/24930747023