software_spec: Add HOMEBREW_BOTTLE_DEFAULT_DOMAIN as a fallback option

[XuehaiPan]

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Add HOMEBREW_BOTTLE_DEFAULT_DOMAIN to downloader mirror list if HOMEBREW_BOTTLE_DOMAIN is other than the default. The downloader may use the HOMEBREW_BOTTLE_DEFAULT_DOMAIN as a fallback option. (resolves #10660)

[MikeMcQuaid]

Thanks! Nice work so far. Some tests would be great (but can wait until we've nailed this down a bit more off desired).

[XuehaiPan]

I think default_root_url could be misleading, because it is not always presented. I renamed it to fallback_root_url. It is not nil when Homebrew::EnvConfig.bottle_domain != HOMEBREW_BOTTLE_DEFAULT_DOMAIN and @root_url is not explicitly modified (Library/Homebrew/dev-cmd/bottle.rb#L405).

[XuehaiPan]

Simply replace the BOTTLE_DOMAIN to BOTTLE_DEFAULT_DOMAIN as the fallback URL. Even if the root URL has been modified explicitly (Library/Homebrew/dev-cmd/bottle.rb#L405). The root URL modification check has been removed. The fallback option only takes effect in fetch.

[MikeMcQuaid]

@XuehaiPan Let me know when there's a version that works OK for you!

[XuehaiPan]

@XuehaiPan Let me know when there's a version that works OK for you!

The current version (@bf87849) works fine for me.

[XuehaiPan]

Probably the final version of this PR.

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @XuehaiPan!