"x.py dist" build fails in llvm when DESTDIR is set #73132

This appears to be the underlying reason for issue #71743.
When I cross-build, I have to use the "dist" x.py target to produce
a bootstrap kit, and in my setup, DESTDIR is aways set (this is in
NetBSD's packaging system, pkgsrc). I have reproduced the problem
outside the pkgsrc setup, and it is indeed a non-null DESTDIR which
triggers this issue.

Such a build errors out with

Dist RLS stage1 (x86_64-unknown-netbsd)
running: "/home/he/src/work/rust/build/x86_64-unknown-netbsd/stage0-tools-bin/fabricate" "generate" "--product-name=Rust" "--rel-manifest-dir=rustlib" "--success-message=RLS-ready-to-serve." "--image-dir" "/home/he/src/work/rust/build/tmp/dist/rls-image" "--work-dir" "/home/he/src/work/rust/build/tmp/dist" "--output-dir" "/home/he/src/work/rust/build/dist" "--non-installed-overlay" "/home/he/src/work/rust/build/tmp/dist/rls-overlay" "--package-name=rls-1.41.0-x86_64-unknown-netbsd" "--legacy-manifest-dirs=rustlib,cargo" "--component-name=rls-preview"
        finished in 15.267
  < Rls { compiler: Compiler { stage: 1, host: "x86_64-unknown-netbsd" }, target: "x86_64-unknown-netbsd" }
  > LlvmTools { target: "x86_64-unknown-netbsd" }
Dist LlvmTools (x86_64-unknown-netbsd)
thread 'main' panicked at 'Error: File "/home/he/src/work/rust/build/x86_64-unknown-netbsd/llvm/bin/llvm-nm" not found!', src/bootstrap/lib.rs:1215:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
        finished in 0.022
Traceback (most recent call last):
...

When DESTDIR is set to $(pwd)/work/destdir, llvm-nm ends up installed in
./work/destdir/home/he/src/work/rust/build/x86_64-unknown-netbsd/llvm/bin/llvm-nm
instead of what the rest of the build expects (ref. above),
./work/rust/build/x86_64-unknown-netbsd/llvm/bin/llvm-nm.
It's as if some part of the build has already figured out the full pathname to the destination,
and then proceeds to prepend $DESTDIR to that absolute path; $(pwd)
is /home/he/src/ in this setup.

The question is whether the rust build system ought to clear DESTDIR
for the build of the internal llvm, at least for the "build" phase?

The workaround I used is to unset DESTDIR for the build in total, but
doing so also for the top-level "build" target has the unfortunate side-effect that
cargo detects this change when "install" time comes, and causes everything
to be re-built. (I can see a workaround for this issue, though.)

As stated in the referenced issue, this problem is not new in 1.43, it also
exists in 1.42 and 1.41.1 -- I did not traverse further back.

The bootstrap used for the 1.41.1 build attempt was:

rustc --version --verbose:

rustc 1.40.0 (73528e339 2019-12-16)
binary: rustc
commit-hash: 73528e339aae0f17a15ffa49a8ac608f50c6cf14
commit-date: 2019-12-16
host: x86_64-unknown-netbsd
release: 1.40.0