try to make cargo-miri work with bootstrap cargo

Author: RalfJung

src/tools/miri/cargo-miri/src/setup.rs

@@ -89,7 +89,10 @@ pub fn setup(
     let cargo_cmd = {
         let mut command = cargo();
         // Allow JSON targets since users do not have a good way to set this flag otherwise.
-        command.arg("-Zjson-target-spec");
+        if !env::var("RUSTC_STAGE").is_ok_and(|s| s == "1") {
+            // ^ is a HACK for bootstrap cargo. FIXME(cfg(bootstrap)) remove the hack.
+            command.arg("-Zjson-target-spec");
+        }
         // Use Miri as rustc to build a libstd compatible with us (and use the right flags).
         // We set ourselves (`cargo-miri`) instead of Miri directly to be able to patch the flags
         // for `libpanic_abort` (usually this is done by bootstrap but we have to do it ourselves).