Update usage of meson-vcs-tag.sh to account for upstream changes

The second argument now specifies the version tag version so let's
adapt. Because the script now supports running without any arguments
at all, let's just do that.

The output now also doesn't use any hyphens anymore so we get rid
of the sed transformation as well;

systemd.spec

@@ -37,7 +37,7 @@ Url:            https://systemd.io
 Version:        255.3
 %else
 # determine the build information from local checkout
-Version:        %(tools/meson-vcs-tag.sh . error | sed -r 's/-([0-9])/.^\1/; s/-g/_g/')
+Version:        %(tools/meson-vcs-tag.sh)
 %endif
 Release:        %autorelease