Support custom github/gitlab domain

[pi0]

From what I have tested so far for private gitlab repo is

1. I need to set export NODE_EXTRA_CA_CERTS=/path/to/cert.pem in .zshrc
2. For example, my private repo is a self hosted gitlab then I need to modify the domain on line 87 & 88 in /giget/shared/gitget.xxxx.mjs
   url: https://gitlab.domain.com/${parsed.repo}/tree/${parsed.ref}${parsed.subdir},
   tar: https://gitlab.domain.com/${parsed.repo}/-/archive/${parsed.ref}.tar.gz

is there anything I can do for gitlab custom domain, for example to set GIGET_GITLAB_URL in .env so that it will pick up custom domain?

3. set GIGET_REGISTRY, GIGET_AUTH in .env

The only downside is I need to modify the gitlab domain myself.

_Originally posted by @hanzoz in nuxt/nuxt#13367

[pi0]

Partially supported by #71.