Rollup merge of #142089 - bjorn3:sysroot_handling_cleanup3, r=petrochenkov

Replace all uses of sysroot_candidates with get_or_default_sysroot

Before this change we had two different ways to attempt to locate the sysroot which are inconsistently used:
* `get_or_default_sysroot` which tries to locate based on the 0th cli argument and if that doesn't work falls back to locating it using the librustc_driver.so location and returns a single path.,
* `sysroot_candidates` which takes the former and additionally does another attempt at locating using `librustc_driver.so` except without linux multiarch handling and then returns both paths.,

The latter was originally introduced to be able to locate the codegen backend back when cg_llvm was dynamically linked even for a custom driver when the `--sysroot` passed in does not contain a copy of cg_llvm. Back then `get_or_default_sysroot` did not attempt to locate the sysroot based on the location of librustc_driver.so yet. Because that is now done, the only case where removing `sysroot_candidates` can break things is if you have a custom driver inside what looks like a sysroot including the `lib/rustlib` directory, but which is missing some parts of the full sysroot like eg rust-lld.

Follow up to rust-lang/rust#138404

Author: workingjubilee