ci: Install GHC/stack via haskell-actions/setup consistently #672

Loadtest Results

param	v12.0.2	main	head
bytes_in.mean	1008	1007	1008
bytes_in.total	28671576	28777331	28633159
bytes_out.mean	3848	3849	3848
bytes_out.total	109478310	109943974	109351305
duration	60009604648	60000594702	60008593708
latencies.50th	951072	943249	942444
latencies.90th	1973400	2055170	2155093
latencies.95th	13532643	13520700	13641287
latencies.99th	13731794	13712541	13822069
latencies.max	25167591	28169966	30883509
latencies.mean	2108504	2099527	2110904
latencies.min	157354	153646	152624
latencies.total	59982729851	59973007652	59981355595
rate	474	476	474
requests	28448	28565	28415
status_codes.200	15518	15580	15500
status_codes.201	5172	5194	5166
status_codes.204	7758	7791	7749
success	1	1	1
throughput	474	476	474
wait	914189	883356	845141