fix: output_cache.go is still using mtime instead of hashing

[zzzz465]

in previous #5 I introduced hash-based caching in discovery_cache.go and artifact_cache.go. however, I missed that I also need to patch output_cache.go which has the most impact on decreasing wire time.

this PR changes mtime based on hashing in output_cache.go, with replacing existing hashing mechanism from standard sha-256, to xx3, which appears to be more performant (note that I/O bottleneck is significant so this won't make any big difference)

also, I need to mention that hash-based equality checks naturally incur I/O, which is way slower than mtime.
in local environment, previous mtime based caching is faster than hashing, which is not possible in CI environment where mtime is always reset.

since now there's three cache (output, artifact, discovery), I did benchmark test on my private repository to see how it works with/without each cache. below is the result.

#	discovery	artifact	output	Time
1	HIT	HIT	HIT	1.5s
2	HIT	HIT	MISS	1.6s
3	HIT	MISS	MISS	18.6s
4	MISS	HIT	MISS	7.1s
5	MISS	MISS	MISS	21.4s

the best bet is to provide an option to choose which one to use key: mtime or hash, so user can select

[cmilesio]

Looks like you've got some issues on this branch, I'm going to need a bit before I can take a look at this caching issue you've ran into.