In the builddeb context, the DEB_HOST_ARCH environment variable is set
to the same value as debian/arch's content, so use the variable with
dpkg-architecture.

This is the last use of the debian/arch file during dpkg-buildpackage time.
Signed-off-by: Bastian Germann <bage@linutronix.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>