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/efiboot/loader/entries/01-archiso-linux.conf

@@ -1,4 +1,4 @@
-title    Arch Linux install medium (%ARCH%, UEFI)
+title    TANINUX install medium (%ARCH%, UEFI)
 sort-key 01
 linux    /%INSTALL_DIR%/boot/%ARCH%/vmlinuz-linux-zen
 initrd   /%INSTALL_DIR%/boot/%ARCH%/initramfs-linux-zen.img

iso/build-profile/efiboot/loader/entries/02-archiso-speech-linux.conf

@@ -1,4 +1,4 @@
-title    Arch Linux install medium (%ARCH%, UEFI) with speech
+title    TANINUX install medium (%ARCH%, UEFI) with speech
 sort-key 02
 linux    /%INSTALL_DIR%/boot/%ARCH%/vmlinuz-linux-zen
 initrd   /%INSTALL_DIR%/boot/%ARCH%/initramfs-linux-zen.img