WIP cross-compilation support

[fasterthanlime]

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[fasterthanlime]

To make this PR thinner, I'd love to get a ✅ on axodotdev/axoasset#212 (and then a cargo publish, and a bump on the version of axoasset that cargo-dist pulls in).

I'd also love to get these merged separately:

• fast profile: c249d81 — this can just be what we make dev be, I'd love that actually, then I wouldn't need to build twice. pretty much, I need the deps to be optimized otherwise we're wasting CPU cycles doing xz compression
• eb14745 — supports a cargo_dist_url_override setting, which lets me serve cargo-dist from, say, https://dl.bearcove.cloud/dump/ in CI — which is how I got this run to finish in 1m48s instead of like, 14m

The rest of the PR is pretty radioactive for now, it has a bunch of .unwrap(), I want to use Triple (the structured version, from target_lexicon) more aggressively, but keep the original (non-parsed) version around, just in case we can't parse it?

My guess is that for now if target_lexicon cannot parse it, chances are we cannot do anything useful with it anyway, but it just sucks to have to parse it again and again just because the thing we're parsing around is a thinly-veiled string.

[fasterthanlime]

Quick piece of insight I had while looking at:

in the absence of other info, we can assume that the custom runners have the arch they're being used for.

Since cargo-dist did not support cross when that config format was invented.

Later, we can disallow doing something like:

[dist.github-custom-runners]
aarch64-unknown-linux-gnu = "my-fast-runner"
x86_64-unknown-linux-gnu = "my-fast-runner"

With something like "what's the target triple for github runner 'my-fast-runner'? tell us by doing this:

[dist.github-custom-runner-specs]
my-fast-runner = { triple = "aarch64-unknown-linux-gnu" }

...or whatever makes sense to us.

[fasterthanlime]

Closing in favor of #1529