releng: Add buildmodes to profiledef.sh

configs/releng/profiledef.sh:
Add a `buildmodes` array to releng's profiledef.sh with the up-to-now default buildmode 'iso'.

configs/releng/profiledef.sh

@@ -7,6 +7,7 @@ iso_publisher="Arch Linux <https://archlinux.org>"
 iso_application="Arch Linux Live/Rescue CD"
 iso_version="$(date +%Y.%m.%d)"
 install_dir="arch"
+buildmodes=('iso')
 bootmodes=('bios.syslinux.mbr' 'bios.syslinux.eltorito' 'uefi-x64.systemd-boot.esp' 'uefi-x64.systemd-boot.eltorito')
 arch="x86_64"
 pacman_conf="pacman.conf"