yices: Download gmp from ftp.gnu.org mirror instead of gmplib.org

This link is much more reliable than the upstream gmplib.org download link, as
gmplib.org firewalls traffic from GitHub Actions IP addresses (see
https://www.theregister.com/2023/06/28/microsofts_github_gmp_project/).

Fixes #44.

.github/ci.sh

@@ -129,7 +129,7 @@ build_yices() {
   mkdir -p install-root/include
   mkdir -p install-root/lib
 
-  (cd repos && curl -o gmp.tar.lz -sL "https://gmplib.org/download/gmp/gmp-6.2.1.tar.lz" && tar xf gmp.tar.lz)
+  (cd repos && curl -o gmp.tar.lz -sL "https://ftp.gnu.org/gnu/gmp/gmp-6.2.1.tar.lz" && tar xf gmp.tar.lz)
 
   pushd repos/gmp-6.2.1
   ./configure $CONFIGURE_FLAGS