tools/gnulib: use git archive as download source

tools/gnulib: use git archive as download source

Snapshot downloads, whether from GNU cgit or GNU gitweb,
are becoming more unreliable and unstable.
Use the GNU gnulib git repository server for downloads
because GNU has disabled cgit server snapshots due to performance issues.

Other possible options include GNU gitweb snapshot or a git bundle download
but these methods either may also have similar performance issues
or require additions to the build system to handle the method
beyond the previous commit.

In case snapshots are to be used again for build tools in the future,
the previous commit reduces the tarball generation attempts
for the original source of snapshots in order to reduce server impact.

A conversation regarding server performance issues and alternatives is linked.

Link: https://lists.gnu.org/archive/html/bug-gnulib/2024-12/msg00124.html
Signed-off-by: Michael Pratt <mcpratt@pm.me>
Link: https://github.com/openwrt/openwrt/pull/16522
Signed-off-by: Robert Marko <robimarko@gmail.com>