Support BASE_URL (or similar) for rolling/green releases

[novas0x2a]

This is an additional feature request on top of #72; we want all of the binary downloads to pull through our transparent mirror. Currently, we're doing that via .bazeliskrc:

BAZELISK_BASE_URL=https://our.mirror/github.com/bazelbuild/bazel/releases/download

But I noticed that rolling and last_green don't use the BASE_URL, so they download directly (which makes sense, given that it would be tricky to use BASE_URL in a way that worked for multiple download types, since they come from different hosts with different path schemes)

Would it be possible to extend the functionality to support this? Not sure the best way, but one way I can think of would be to add a key to BAZELISK_FORMAT_URL that contains the URL that bazelisk would have used anyway, like maybe:

• %H means the hostname (that would have been used if FORMAT_URL wasn't set)
• %P means the path (same)

So then if I set BAZELISK_FORMAT_URL to https://our.mirror/%H/%P (and the DownloadRolling / DownloadAtCommit supported it) then I think that would do what I wanted?