add FIXME to replace this env var in the future

Author: RalfJung

src/bootstrap/sanity.rs

@@ -156,6 +156,9 @@ than building it.
         }
 
         // Some environments don't want or need these tools, such as when testing Miri.
+        // FIXME: it would be better to refactor this code to split necessary setup from pure sanity
+        // checks, and have a regular flag for skipping the latter. Also see
+        // <https://github.com/rust-lang/rust/pull/103569#discussion_r1008741742>.
         if env::var_os("BOOTSTRAP_SKIP_TARGET_SANITY").is_some() {
             continue;
         }
@@ -218,6 +221,9 @@ than building it.
         }
 
         // Some environments don't want or need these tools, such as when testing Miri.
+        // FIXME: it would be better to refactor this code to split necessary setup from pure sanity
+        // checks, and have a regular flag for skipping the latter. Also see
+        // <https://github.com/rust-lang/rust/pull/103569#discussion_r1008741742>.
         if env::var_os("BOOTSTRAP_SKIP_TARGET_SANITY").is_some() {
             continue;
         }