Fix rustfmt relative custom command

When `rustfmt.overrideCommand` is a relative path, it's joined with the
workspace root to make it an absolute path. Without this the command
path can't be resolved, especially because rustfmt changes the cwd to
make sure rustfmt.toml works correctly.

Currently `run_rustfmt` only does this when it finds a `target_spec`
using `target_spec_for_file` which only works for the root file of the
target.

This commit changes it to use `TargetSpec::from_file` which works for
any file in the crate.

src/tools/rust-analyzer/crates/rust-analyzer/src/handlers/request.rs

@@ -2442,8 +2442,7 @@ fn run_rustfmt(
         }
         RustfmtConfig::CustomCommand { command, args } => {
             let cmd = Utf8PathBuf::from(&command);
-            let target_spec =
-                crates.first().and_then(|&crate_id| snap.target_spec_for_file(file_id, crate_id));
+            let target_spec = TargetSpec::for_file(snap, file_id).ok().flatten();
             let extra_env = snap.config.extra_env(source_root_id);
             let mut cmd = match target_spec {
                 Some(TargetSpec::Cargo(_)) => {