Download protoc from custom repositories

[rafan]

When configuring protoc using artifact like the following:

protobuf {
    protoc {
        artifact = 'com.google.protobuf:protoc:3.21.12'
    }
}

It always downloads from the Maven Central (i.e., repo.apache.maven.org). However, it fails in a restricted environment where egress connections are regulated.

Either the plugin honors the repositories setup in build.gradle or it allows customization on repositories.