bug#40726: scandir* error on armhf

[Julien Lepiller]

Hi Guix!

I recently tried to build guix from git on my armhf machine. However,
it failed at 14% when loading gnu/system/installer.scm.  The error was:

error: failed to load 'gnu/system/install.scm': Nom de fichier trop
long

(name too long)

Indeed, stracing the build process I see a lstat64 syscall on this
file: ~/guix/repo/master/gnu/../gnu/installer//////////////////...
(with many /).

Reverting commit 82d8959e5d137b2061a68878d78a8f74a238ac44 lets me build
guix from git.  I tried to run guix pull too, in order to test from a
more recent system, but guix-modular-master fails on ci since around
that commit, and the same path is shown in the log, for instance:

https://ci.guix.gnu.org/log/7jzm70m187xdb41jmljmq6vkzjwsnh2d-guix-system

ends with a lstat of
/gnu/store/qi99vg92icmpcw623rvg8rqdlvilqb00-guix-system-source/gnu/../gnu/installer////////...

other architectures don't seem to have that issue, as ci is able to
build modular-master for them.  I was also able to build guix from git
on my x86_64 machine.