go-fuzz-build: Build only when out of date

[tv42]

Creating the *-fuzz.zip takes a lot of time even on a simple project. Yet simple checks like "only create if it doesn't exist" easily leave us fuzzing already-fixed code. It'd be really nice if it behaved like go install and friends and only recreated the zip if the inputs have updated. I'm sure implementing this once in go-fuzz is better than every caller implementing a bad version of it in a shell script, which is what I almost just did...

[dvyukov]

I agree that it would be nice. Pull request is welcome.

I think eventually go-fuzz instrumentation should be merged into Go compiler, and go-fuzz-build logic should be merged into go tool. That would solve lots of issues including this one.

[dgryski]

WIll be obsoleted by #154

[josharian]

I've made considerable progress on build times. When cmd/go's -trimpath plays nicely with the cache (golang/go#31896), it'll get even better. I'm going to call this fixed for now.