root: determine_variants cxxstd=20 support (#32673)

This is somewhat ahead of reality, but once/when external ROOT installs with cxxstd=20 are common, we will need this to pick up the correct variant.

var/spack/repos/builtin/packages/root/package.py

@@ -334,6 +334,8 @@ def determine_variants(cls, exes, version_str):
             v.append("cxxstd=14")
         elif "cxx17" in f:
             v.append("cxxstd=17")
+        elif "cxx20" in f:
+            v.append("cxxstd=20")
 
         # helper function: check if featurename is in features, and if it is,
         # append variantname to variants. featurename may be a list/tuple, in