Patch for local builds of MadGraph

Zach Marshallrequested to merge
zmarshal/athena:21.6_MGC_Patch into 21.6

This patch protects users using local builds of MadGraph. The old version raised an error because it assumed that the path to MadGraph in use had 'MCGenerators' in it, which of course it needn't if it's a local installation. This version should be safer.

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