iso: fresh work dir each build (avoid stale-stage reuse) + modern bootmodes

mkarchiso reuses completed stages in the work dir -> a 'too short' build that
ships a stale airootfs. bootstrap now 'rm -rf' the work dir first. Also switch
profiledef bootmodes to the non-deprecated names (bios.syslinux, uefi.systemd-boot).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

iso/overrides/profiledef.sh

@@ -8,8 +8,7 @@ iso_application="TANINUX (camel) — niri/eww desktop · live + Calamares instal
 iso_version="$(date +%Y.%m.%d)"
 install_dir="tanin"
 buildmodes=('iso')
-bootmodes=('bios.syslinux.mbr' 'bios.syslinux.eltorito'
-           'uefi-x64.systemd-boot.esp' 'uefi-x64.systemd-boot.eltorito')
+bootmodes=('bios.syslinux' 'uefi.systemd-boot')
 arch="x86_64"
 pacman_conf="pacman.conf"
 airootfs_image_type="squashfs"