fix(ci): ensure /dev/fd symlinks exist for mkinitcpio and procsub

The chroot's /dev only gains fd/stdin/stdout/stderr symlinks at boot, and
container executors often lack them, which breaks mkinitcpio's preset step
(pipes through /dev/stdin) and bash process substitution
("/dev/fd/63: No such file or directory").

- build-iso workflow: create the links in the executor's /dev, then mount a
  fresh devtmpfs and create them there too so the chroot tree is covered
  whether the executor's /dev is devtmpfs or a plain tmpfs
- customize_airootfs.sh: self-heal the same links inside the chroot

airootfs/root/customize_airootfs.sh

@@ -1,6 +1,19 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+# The chroot's /dev is the kernel's single shared devtmpfs, which only gets the
+# fd/stdin/stdout/stderr symlinks at boot (from mkinitcpio's init). If the
+# runner environment lacks them, process substitution (< <(...)) and upstream
+# tools (pacman-key) fail with "… /dev/fd/63: No such file or directory".
+# Self-heal here; the CI build-iso workflow does the same before pacstrap so
+# that mkinitcpio's hook (which pipes through /dev/stdin) works too.
+for _link in "fd:/proc/self/fd" "stdin:/proc/self/fd/0" \
+  "stdout:/proc/self/fd/1" "stderr:/proc/self/fd/2"; do
+  [[ -e "/dev/${_link%%:*}" || -L "/dev/${_link%%:*}" ]] ||
+    ln -sfn "${_link#*:}" "/dev/${_link%%:*}"
+done
+unset _link
+
 # Populate the keyring in the built root from the keyring bundles
 # (/usr/share/pacman/keyrings, incl. the copied cachyos keyring).
 # This mirrors the guide; it runs inside the image you are making.