What do you call someone who is always looking for loopholes?

As someone who is always looking to find their way out of or through a situation with the use of loopholes.

It would depend on context, but I'd use the term "rules lawyer".

Yes, the word for "one who circumvents", one who evades or outwits by cunning methods or strategies, is a "circumventor" (source: Webster's Revised Unabridged Dictionary, 1913 Edition).

I'm not sure if this word is obsolete today though.

If you're looking for a noun, I'd suggest 'sophist', meaning "one skilled in elaborate and devious argumentation." It can be turned into 'sophistry' if you're looking to describe the act of looking for the loophole.

If you're looking for an adjective to describe such an individual, 'guileful' or 'calculating' do the job nicely. (Meaning "deceitfully or treacherously cunning" and "coldly scheming or conniving" respectively.)