Set RUSTC_BOOTSTRAP=1 in the environment so that stable versions of rustc
allow the use of -Z* compiler options, rather than erroring out with

  error: the option `Z` is only accepted on the nightly compiler

--- a/build/rust/gni_impl/rustc_wrapper.py
+++ b/build/rust/gni_impl/rustc_wrapper.py
@@ -249,6 +249,9 @@ def main():
     env[k] = v
     fixed_env_vars.append(k)
 
+  # Debian: Allow -Z* compiler options
+  env['RUSTC_BOOTSTRAP'] = '1'
+
   try:
     if args.v:
       print(' '.join(f'{k}={shlex.quote(v)}' for k, v in env.items()),