activation: Fix ‘mkdir-p/perms’ on the Hurd.

Fixes <https://issues.guix.gnu.org/77992>.

Fixes a bug whereby ‘mkdir-p/perms’ would throw an exception on the
Hurd:

  In gnu/build/activation.scm:
      97:20  1 (mkdir-p/perms _ #("ludo" "x" 1000 998 "Ludovic…" …) …)
  In unknown file:
	     0 (open "." 7340032 #<undefined>)

  ERROR: In procedure open:
  In procedure scm_fdes_to_port: requested file mode not available on fdes

This, in turn, would cause ‘user-homes’ to fail to start on the Hurd
since commit da741d8931.

* gnu/build/activation.scm (mkdir-p/perms): Add O_RDONLY to ‘open-flags’.

Reported-by: Yelninei <yelninei@tutamail.com>
Change-Id: I74f90599338772ba4341067215b864273aa30b3e

gnu/build/activation.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012-2021, 2025 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>
 ;;; Copyright © 2013 Andreas Enge <andreas@enge.fr>
 ;;; Copyright © 2015, 2018 Mark H Weaver <mhw@netris.org>
@@ -90,6 +90,7 @@ and bits are set according to the default behaviour of 'mkdir'."
   ;; By combining O_NOFOLLOW and O_DIRECTORY, this procedure automatically
   ;; verifies that no components are symlinks.
   (define open-flags (logior O_CLOEXEC ; don't pass the port on to subprocesses
+                             O_RDONLY  ;needed on the Hurd, harmless on Linux
                              O_NOFOLLOW ; don't follow symlinks
                              O_DIRECTORY)) ; reject anything not a directory