feat(toolchain): consider external rust-analyzer when calling a proxy

[rami3l]

Closes #3846.

Following the reasoning in #3846 (comment), this PR make rustup prioritize rustup-hosted RA and only delegates to the outside as a final rescue. This way, it will also close #3299:

> rust-analyzer --version
info: `rust-analyzer` is unavailable for the active toolchain
info: falling back to "~/.local/share/nvim/mason/bin/rust-analyzer"
rust-analyzer 0.3.2449-standalone (3b57c00151 2025-05-04)

This approach has also been approved by @davidbarsky in #t-rustup > More frequent rust-analyzer updates in rustup @ 💬 as a temporary measure before the new update cadence for RA on our release server is settled.

Concerns

• Should we emit a log line indicating the fallback is happening? Idem for the pre-existing cargo fallback. Added.