fix building sysroot for JSON targets

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

@@ -88,6 +88,8 @@ 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");
         // 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).

src/tools/miri/ci/ci.sh

@@ -129,7 +129,7 @@ function run_tests_minimal {
   time ./miri test $TARGET_FLAG "$@"
 
   # Ensure that a small smoke test of cargo-miri works.
-  time cargo miri run --manifest-path test-cargo-miri/no-std-smoke/Cargo.toml $TARGET_FLAG
+  time cargo miri run --manifest-path test-cargo-miri/no-std-smoke/Cargo.toml -Zjson-target-spec $TARGET_FLAG
 
   endgroup
 }