Rollup merge of #148709 - karolzwolak:bootstrap-override-lld-should-disable-self-contained-lld, r=Mark-Simulacrum

fix: disable self-contained linker when bootstrap-override-lld is set

Part of rust-lang/rust#148708.

Makes `rust.bootstrap-override-lld` set to `true` or `"external"` disable self-contained linker — actually using linker from one's system.

Author: matthiaskrgr