GnuTests: Required automake version is too new

[oech3]

When I switched to tarball, newer automake was introduced at

coreutils/.github/workflows/GnuTests.yml

Lines 72 to 73 in 5fd3459

	curl http://launchpadlibrarian.net/831710181/automake_1.18.1-3_all.deb > automake-1.18.deb
	sudo dpkg -i --force-depends automake-1.18.deb

which might problematic dep for tester on current stable Ubuntu. (And causing network err...)

Building from tarball instead of git reduces many build deps and many steps. So I'd like to allow to use system OOD automake keeping tarball if possible.