help users set a macos sdk deployment target

[Gankra]

Even the newest macos sdks all have support for much older "deployment targets" (you can tell the 14 sdk to build for 10.12, for instance). This is governed by MACOSX_DEPLOYMENT_TARGET env var. At best, cargo-dist should default to automatically setting this to a reasonable value. At worst we should have docs telling people to set this in their cargo-dist env-vars (and we should test it works! i have a macos 11 intel macbook to test with).

Doing this would unblock us unabashedly using the apple silicon runners, which would be a huge win.

A former apple compiler engineer who now does a lot of rust dev has informed me that this env-var has a ton of confusing discussion, and that the value rustc "defaults" to is just for the stdlib/itself, seemingly rust binaries you build default to deployment_target == sdk_version which is of course a portability nightmare.

For reference, rustc defaults to 10.12 for itself (rustc --target x86_64-apple-darwin --print deployment-target), which is super generous.

See also this (confusing) issue about how cargo doesn't set the env-var for build scripts, which makes cdeps very sad: rust-lang/cargo#13115