github: Cache the gnu make tarball

This avoids breaking the build if the gnu download server is
unavailable.

build-make.sh

@@ -52,7 +52,9 @@ download() {
 }
 
 if [ ! -d make-$MAKE_VERSION ]; then
-    download https://ftpmirror.gnu.org/gnu/make/make-$MAKE_VERSION.tar.gz
+    if [ ! -e make-$MAKE_VERSION.tar.gz ]; then
+        download https://ftpmirror.gnu.org/gnu/make/make-$MAKE_VERSION.tar.gz
+    fi
     tar -zxf make-$MAKE_VERSION.tar.gz
 fi