Win32 git: fetch potentially new tags before running the tags build

Apparently tags are not copied over automatically using git pull.
This may be a side effect of our main repo still being svn, I'm not sure.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22981 57a11ea4-9604-0410-9ed3-97b8803252fd