Build: spec: fix broken pre-release by tag detection

The breakage was introduced in 2d6e6c374 (avoid bashism due to
rpmbuild-popen-/bin/sh chain) which accidentally tried to access RPM
macro level variable ("%{s}") instead of the one in the intended scope
of the nested shell ("${s}"}. This simple swap can then be further
simplified to refer to the underlying macro level variable right away,
so carry this out as well.