Scan code with govulncheck

[alexandear]

This PR extends CI workflow with vulnerability scanning using govulncheck.

Govulncheck is a command-line tool that helps Go users find known vulnerabilities in their project dependencies. The tool can analyze both codebases and binaries, and it reduces noise by prioritizing vulnerabilities in functions that your code is actually calling.

Govulncheck is run using govulncheck-action.