In a test project where I just combined RUSTC_BOOTSTRAP=1 together with a -Z threads=8 option, I’ve noticed that the error might be cached and the RUSTC_BOOTSTRAP=1 not recognized as a significant change to trigger actually retrying the build. Try to clean first, or otherwise force a re-build to actually be tried (e.g. touching a relevant source file, or manually deleting relevant artifacts).
Edit: Just noticing, in my test, it was actually sufficient to slightly alter RUSTFLAGS, e.g. by changing between -Zthreads=8 without space and -Z threads=8 with a space, to make it re-try properly.