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/root/.bash_profile

@@ -1,6 +0,0 @@
-# Live ISO: on the first console (tty1), launch the TANINUX installer.
-# (On an installed system this file isn't present — Calamares' shellprocess
-#  strips the live-only bits from the target.)
-if [[ "$(tty)" == /dev/tty1 ]] && [[ -z "${WAYLAND_DISPLAY:-}" ]] && ! pgrep -x cage >/dev/null 2>&1; then
-  exec tanin-installer
-fi

iso/build-profile/airootfs/root/.zprofile

@@ -0,0 +1,5 @@
+# Live ISO: auto-launch the TANINUX installer on the first console.
+# (Removed from the installed target by Calamares' shellprocess-striplive.)
+if [[ "$(tty)" == /dev/tty1 ]] && [[ -z "$WAYLAND_DISPLAY" ]] && ! pgrep -x cage >/dev/null 2>&1; then
+  exec tanin-installer
+fi