Fix ISO build failure by correcting bootmodes format

The ISO build was failing with the error `_make_bootmode_: command not found` because the `bootmodes` variable in `profiledef.sh` was defined as a bash array. The `mkarchiso` script expects a comma-separated string for the boot modes.

This commit changes the `bootmodes` variable to a single comma-separated string, which resolves the build error.

Author: google-labs-jules[bot]

profiledef.sh

@@ -8,9 +8,7 @@ iso_application="Arch Linux No Beep Live/Rescue DVD"
 iso_version="$(date --date="@${SOURCE_DATE_EPOCH:-$(date +%s)}" +%Y.%m.%d)"
 install_dir="arch"
 buildmodes=('iso')
-bootmodes=('bios.syslinux.mbr' 'bios.syslinux.eltorito'
-           'uefi-ia32.systemd-boot.esp' 'uefi-x64.systemd-boot.esp'
-           'uefi-ia32.systemd-boot.eltorito' 'uefi-x64.systemd-boot.eltorito')
+bootmodes=('bios.syslinux.mbr,bios.syslinux.eltorito,uefi-ia32.systemd-boot.esp,uefi-x64.systemd-boot.esp,uefi-ia32.systemd-boot.eltorito,uefi-x64.systemd-boot.eltorito')
 arch="x86_64"
 pacman_conf="pacman.conf"
 airootfs_image_type="squashfs"