diagnostic: guard cache check on Jenkins too

[DomT4]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
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Apparently Jenkins doesn't set CI in the env automatically? This should get CI working again. Sorry!

[MikeMcQuaid]

@DomT4 Thanks for the quick fix. Merging for now but I want to tweak this so we don't have to keep remembering to set JENKINS_HOME here.

[DomT4]

Maybe Utils.is_a_robot? or something that combines these variables into a blob, since we're using it more than once now.

[DomT4]

Thanks for merging. Apologies for this one; was sure Jenkins set CI by default sigh. Maybe I was misremembering and thinking of Travis, which is possible.