Auto merge of #13200 - P1n3appl3:sysroot, r=Veykril
Auto merge of #13200 - P1n3appl3:sysroot, r=Veykril

Use proc-macro-srv from sysroot in rust-project.json workspaces

This was discussed [on zulip](https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer/topic/.60sysroot.60.20vs.20.60sysroot_src.60.20for.20.60rust-project.2Ejson.60.3F/near/293023920), basically in `rust-project.json` workspaces RA doesn't respect the `sysroot` setting when picking which `proc-macro-srv` to launch, and this causes abi mismatches in practice.

This is the simple fix that `@Veykril` suggested, and I've verified that it works on Fuchsia by inspecting the cmdline with `pgrep rust-analyzer | xargs ps -fp` to check that it's using the `proc-macro-srv` from our prebuilts which matches the `sysroot` specified in our `rust-project.json`.

Can this be merged as is, or do we need to add tests that exercise this?

rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer