archiso/mkarchiso: clear environment variables when chrooting

Environment variables can interfere with the commands in chroot.
This causes issues when the environment variables specify a path that does not exist inside the chroot, e.g. if TMPDIR is set to a custom value.

Fixes https://bugs.archlinux.org/task/70580 .

archiso/mkarchiso

@@ -324,13 +324,12 @@ _make_packages() {
 
     if [[ -n "${gpg_key}" ]]; then
         exec {ARCHISO_GNUPG_FD}<>"${work_dir}/pubkey.gpg"
-        export ARCHISO_GNUPG_FD
     fi
 
     if [[ "${quiet}" = "y" ]]; then
-        pacstrap -C "${work_dir}/${buildmode}.pacman.conf" -c -G -M -- "${pacstrap_dir}" "${buildmode_pkg_list[@]}" &> /dev/null
+        env -i "ARCHISO_GNUPG_FD=${ARCHISO_GNUPG_FD}" pacstrap -C "${work_dir}/${buildmode}.pacman.conf" -c -G -M -- "${pacstrap_dir}" "${buildmode_pkg_list[@]}" &> /dev/null
     else
-        pacstrap -C "${work_dir}/${buildmode}.pacman.conf" -c -G -M -- "${pacstrap_dir}" "${buildmode_pkg_list[@]}"
+        env -i "ARCHISO_GNUPG_FD=${ARCHISO_GNUPG_FD}" pacstrap -C "${work_dir}/${buildmode}.pacman.conf" -c -G -M -- "${pacstrap_dir}" "${buildmode_pkg_list[@]}"
     fi
 
     if [[ -n "${gpg_key}" ]]; then
@@ -372,7 +371,7 @@ _make_customize_airootfs() {
         _msg_info "Running customize_airootfs.sh in '${pacstrap_dir}' chroot..."
         _msg_warning "customize_airootfs.sh is deprecated! Support for it will be removed in a future archiso version."
         chmod -f -- +x "${pacstrap_dir}/root/customize_airootfs.sh"
-        eval -- arch-chroot "${pacstrap_dir}" "/root/customize_airootfs.sh"
+        eval -- env -i arch-chroot "${pacstrap_dir}" "/root/customize_airootfs.sh"
         rm -- "${pacstrap_dir}/root/customize_airootfs.sh"
         _msg_info "Done! customize_airootfs.sh run successfully."
     fi