fix(bootstrap): also unset MFLAGS when fifo jobserver not present

CI jobs were sometimes hanging because cargo would inherit the jobserver based the MFLAGS
environment variable when it contained file descriptors. However, these FDs were not
valid, since bootstrap.py does not pass them through from make.

This leads cargo's job server to use invalid FDs when setting up the
jobserver.

Bootstrap had a mitigation for this when it came from MAKEFLAGS,
but not when it came from MFLAGS.

Author: arlosi

src/bootstrap/src/core/builder/cargo.rs

@@ -566,6 +566,7 @@ impl Builder<'_> {
 
         if !has_modern_jobserver {
             cargo.env_remove("MAKEFLAGS");
+            cargo.env_remove("MFLAGS");
         }
 
         cargo