Use old release version of gmp again

Their hg server is way too flakey to rely on, download a release tarball
from GNU mirror instead.

I hope no important fixes are missed due to the loss of update
automation this causes.

scripts.d/25-gmp.sh

@@ -1,17 +1,18 @@
 #!/bin/bash
 
-SCRIPT_REPO="https://gmplib.org/repo/gmp/"
-SCRIPT_HGREV="614a1cd8bb1d"
+SCRIPT_VERSION="6.2.1"
+SCRIPT_SHA512="c99be0950a1d05a0297d65641dd35b75b74466f7bf03c9e8a99895a3b2f9a0856cd17887738fa51cf7499781b65c049769271cbcb77d057d2e9f1ec52e07dd84"
+SCRIPT_URL="https://ftp.gnu.org/gnu/gmp/gmp-${SCRIPT_VERSION}.tar.xz"
 
 ffbuild_enabled() {
     return 0
 }
 
 ffbuild_dockerbuild() {
-    retry-tool sh -c "rm -rf gmp && hg clone -r '$SCRIPT_HGREV' -u '$SCRIPT_HGREV' '$SCRIPT_REPO' gmp"
-    cd gmp
+    retry-tool check-wget "gmp.tar.xz" "$SCRIPT_URL" "$SCRIPT_SHA512"
 
-    ./.bootstrap
+    tar xaf "gmp.tar.xz"
+    cd "gmp-$SCRIPT_VERSION"
 
     local myconf=(
         --prefix="$FFBUILD_PREFIX"