Cleanup: make the `self-contained` path within sysroot calculation more principled

See https://github.com/rust-lang/rust/pull/142357#discussion_r2139930429.

Suggestion (follow-up, not for this PR): maybe pull out a method to do the "path to sysroot target self-contained" logic.

...

I actually had a commit for that, but it's super tricky (lol). Because:

• There are like 5 places in bootstrap that access the self-contained directory. Some of them need a relative path, some of them absolute, some of them work with a compiler, some don't, it's a mess.
• The functions that we have in Builder for getting similar paths (invoked through the Libdir step) actually delete the directory before returning it to you 🤦 I tried to use it here and it broke everything 😂 I need to clean all of this up eventually, and move the directory clearing to a single step, so that you can get a path to something without bootstrap falling over, lol.

-- https://github.com/rust-lang/rust/pull/142357#discussion_r2139952945