Add -Z unstable-options to rustc invocation to make --target work

[FabianWolff]

Currently when using a custom target JSON without Cargo, there is the following problem:

2026-03-04T13:41:29.078675707Z  WARN failed to get rustc cfgs e=unable to fetch cfgs via `"/.../rustc" "--print" "cfg" "-O" "--target" "/.../custom_target.json"`

Caused by:
    "/.../rustc" "--print" "cfg" "-O" "--target" "/.../custom_target.json" failed, exit status: 1
    stderr:
    error: error loading target specification: custom targets are unstable and require `-Zunstable-options`
      |
      = help: run `rustc --print target-list` for a list of built-in targets

Just a few lines above, we already set -Z unstable-options in the Cargo branch:

rust-analyzer/crates/project-model/src/toolchain_info/rustc_cfg.rs

Line 69 in 2a8f00f

cmd.args(["rustc", "-Z", "unstable-options"]).args(RUSTC_ARGS);

We should do the same when calling rustc directly.

[Veykril]

That will break the command for all stable toolchains

[Veykril]

We could heuristically check if the target ends with .json, emit this option + adding RUSTC_BOOTSTRAP=1 to the environment