HEP: set URL fetch method to curl with retry (spack#2863)

* HEP: set URL fetch method to curl with retry

* HEP: fix comment

Author: wdconinc

stacks/hep/spack.yaml

@@ -7,6 +7,10 @@ spack:
   concretizer:
     static_analysis: true
 
+  config:
+    # ROOT fails downloads frequently
+    url_fetch_method: curl --continue-at - --retry 3 --retry-all-errors
+
   packages:
     all:
       target: ["x86_64_v3"]