CI: change ci runners type

[sergejparity]

Closes #495

[paritytech-cicd-pr]

BENCHMARKS

	NATIVE			WASMTIME
BENCHMARK	MASTER	PR	DIFF	MASTER	PR	DIFF	WASMTIME OVERHEAD
execute/ count_until	886.74µs	886.50µs	⚪ -0.07%	2.38ms	2.38ms	⚪ 0.09%	🔴 169%
execute/ factorial_iterative	400.14ns	408.24ns	🔴 1.68%	992.43ns	997.05ns	⚪ 0.57%	🔴 144%
execute/ factorial_recursive	762.18ns	759.61ns	⚪ -0.70%	1.55µs	1.56µs	⚪ 0.55%	🔴 106%
execute/ fib_iterative	1.79ms	1.79ms	⚪ -0.04%	5.12ms	5.11ms	⚪ -0.14%	🔴 185%
execute/ fib_recursive	6.79ms	6.89ms	🔴 1.46%	14.04ms	14.03ms	⚪ -0.11%	🔴 104%
execute/ global_bump	1.44ms	1.43ms	⚪ -0.14%	3.63ms	3.66ms	⚪ 0.69%	🔴 155%
execute/ host_calls	32.84µs	32.02µs	⚪ -4.56%	45.08µs	45.06µs	⚪ -0.06%	🟢 41%
execute/ memory_fill	1.53ms	1.54ms	⚪ 0.16%	4.33ms	4.33ms	⚪ 0.00%	🔴 182%
execute/ memory_sum	1.55ms	1.52ms	⚪ -2.12%	4.36ms	4.53ms	🔴 4.06%	🔴 199%
execute/ memory_vec_add	3.02ms	3.02ms	⚪ 0.20%	9.03ms	8.94ms	⚪ -0.90%	🔴 196%
execute/ recursive_is_even	1.28ms	1.29ms	⚪ 0.27%	2.53ms	2.60ms	🔴 2.43%	🔴 101%
execute/ recursive_ok	172.83µs	172.56µs	⚪ 0.12%	358.31µs	364.81µs	⚪ 1.82%	🔴 111%
execute/ recursive_scan	211.72µs	213.04µs	⚪ 0.62%	452.80µs	460.70µs	🔴 1.59%	🔴 116%
execute/ recursive_trap	17.29µs	17.31µs	⚪ -0.11%	34.89µs	34.76µs	⚪ -0.39%	🔴 101%
execute/ regex_redux	654.17µs	650.18µs	⚪ -0.49%	1.60ms	1.60ms	⚪ -0.38%	🔴 145%
execute/ rev_complement	586.95µs	595.73µs	⚪ 1.38%	1.56ms	1.55ms	⚪ -0.41%	🔴 160%
execute/ tiny_keccak	473.40µs	446.09µs	🟢 -5.72%	1.41ms	1.44ms	🔴 1.97%	🔴 223%
execute/ trunc_f2i	1.07ms	1.07ms	⚪ -0.45%	2.72ms	2.72ms	⚪ -0.02%	🔴 155%
instantiate/ wasm_kernel	69.67µs	68.91µs	⚪ -2.48%	126.64µs	95.40µs	🟢 -24.76%	🟢 38%
translate/ wasm_kernel	4.84ms	4.83ms	⚪ -0.20%	9.41ms	9.47ms	⚪ 0.74%	🟡 96%

Link to pipeline

[Robbepop]

Thanks a lot @sergejparity for the research work behind this PR!
The new numbers look good, especially with respect to low variability.