5651: Add track_env_var to the proc macro server r=kjeremy a=lnicola
See https://github.com/rust-lang/rust/pull/74653.
Fixes#6054.
Fixes#5640, maybe.
Should be merged when 1.47 is released.
Proc macros still don't work for me, but it no longer crashes.
Co-authored-by: Laurențiu Nicola <lnicola@dend.ro>