teach script/integration how to run against a specific git-lfs build

test/testhelpers.sh

@@ -102,14 +102,14 @@ setup() {
   rm -rf "$REMOTEDIR"
   mkdir "$REMOTEDIR"
 
-  if [ -z "$SKIPCOMPILE" ]; then
+  if [ -z "$SKIPCOMPILE" ] && [ -z "$LFS_BIN" ]; then
     echo "compile git-lfs for $0"
     script/bootstrap || {
       return $?
     }
   fi
 
-  echo "Git LFS: $(which git-lfs)"
+  echo "Git LFS: ${LFS_BIN:-(which git-lfs)}"
   git lfs version
 
   if [ -z "$SKIPCOMPILE" ]; then