Can I repurpose an HVAC circuit for shop outlets?
Short answer is yes, but... You might want to consider a small sub panel (outdoor rated) there if you have a 4 wire service to that location. That way, if your needs change (IE: grow), you'll have the power there.   There are lots of posts here regarding connecting a sub-panel... esp. the importance of separating the neutral from the ground in a sub-panel. 

If you don't want the sub-panel, you'll probably need a junction box of some sort to connect the 8 ga to 12 ga. If aluminum you'll need special connectors and noalox compound on the AL.   In the main panel, you'll need to splice wires and downsize the breaker to 20 amps. 

I think I got this right, but one of the "big three" might be able to improve on this answer.