ci: bump actions/checkout to v4 in build.yml

[rubenhensen]

Summary

• Bumps actions/checkout from @v3 to @v4 in the test and format jobs
• Bumps actions/checkout from @v2 to @v4 in the test-wasm-browsers job
• Eliminates deprecated Node 16 runners; aligns with delivery.yml (already on @v6)

Closes #157.

This is the diff that was blocked from being pushed by the dobby agent because its GitHub App installation lacks the workflows: write permission. Pushed manually with a PAT carrying the workflow scope.

Test plan

• CI is green on this branch (test, format, test-wasm-browsers)