Bump installer image rootfs size on Fedora 30 update tests

https://bodhi.fedoraproject.org/updates/FEDORA-2020-1070052d10#comment-1284223
It seems the rootfs on the Fedora 30 installer images we build at
present has gotten very big, so big that an update which contains
some very slightly larger firmware packages causes the rootfs to
be completely full (though lorax doesn't fail) and the image
doesn't boot.

I don't yet know when or why the rootfs got that big, but it's
not really a bug in this update, so for now let's just tell
lorax to use a bigger rootfs so the tests pass for this and any
similar future updates, until I can maybe find time to pinpoint
the culprit more precisely.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

tests/_installer_build.pm

@@ -16,6 +16,10 @@ sub run {
     assert_script_run "pushd /root/imgbuild";
     assert_script_run "setenforce Permissive";
     my $cmd = "lorax -p Fedora -v ${version} -r ${version} --repo=/etc/yum.repos.d/fedora.repo";
+    # rootfs in F30 installer images seems to have got very big at
+    # some point, let's work around that for now:
+    # https://bodhi.fedoraproject.org/updates/FEDORA-2020-1070052d10#comment-1284223
+    $cmd .= " --rootfs-size 3" if ($version eq 30);
     unless (get_var("DEVELOPMENT")) {
         $cmd .= " --isfinal --repo=/etc/yum.repos.d/fedora-updates.repo";
     }