Refactor Helm installation script to use local GPG keys file

[KarstenB]

• Removed the download of GPG keys from GitHub.
• Eliminated the function for checking reachable GPG key servers.
• Updated the script to copy a local KEYS file instead.

This approach eliminates pgp.com server down issue like #1272 and can sometimes improve build performance. The only downside is that the KEYS file needs to be updated when upstream updates it. But a quick check of https://github.com/helm/helm/commits/main/KEYS shows that those are very infrequent.