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/build-profile/airootfs/etc/calamares/modules/shellprocess-striplive.conf

@@ -6,6 +6,6 @@ timeout: 30
 script:
 - "rm -f /etc/systemd/system/getty@tty1.service.d/autologin.conf"
 - "rmdir --ignore-fail-on-non-empty /etc/systemd/system/getty@tty1.service.d 2>/dev/null || true"
-- "rm -f /root/.bash_profile"
+- "rm -f /root/.zprofile"
 - "rm -f /usr/local/bin/tanin-installer"
 - "systemctl set-default graphical.target"