Fixes <https://bugs.gnu.org/42558>.
Until now, loading 'pack-audit.so' in a truly non-Guix environment would
usually fail because 'pack-audit.so' depends on 'libgcc_s.so' and
'libc.so', none of which could be found.
Furthermore, the test was not working as expected: the trick
unshare -mrf sh -c 'mount -t tmpfs none /gnu ; ...'
would allow the fakechroot engine to make its store available as
/gnu/store as a result of another bug.
* gnu/packages/aux-files/run-in-namespace.c (relocated_search_path): New
function.
(exec_with_loader): Pass "--library-path" to the loader.
* guix/scripts/pack.scm (wrapped-package)[build](runpath): New procedure.
(elf-loader-compile-flags): Pass "-DLOADER_AUDIT_RUNPATH".
* tests/guix-pack-relocatable.sh: Remove 'STORE_PARENT'.
(run_without_store): New function. Erase $NIX_STORE_DIR instead of
$STORE_PARENT.
Use 'run_without_store' throughout.
c6c0d5a22c