Change base url to a custom mirror

[mandeep-singh-ck]

I wanted to run swiftly in a air gapped CI environment where there is no direct internet access. I want to set up an Artifactory instance to mirror download.swift.org.

Can we set a environment variable that changes base url that swiftly uses to download dependencies?

In this scenario all the gpg signatures / checksums would be unaffected and still would be verified against the source of truth (swift.org).

The benefit would be:

1. Dependencies download faster in my CI environment
2. Reduce bandwidth usage on the download.swift.org site.