Use a single cache for global and non-global binaries

[douglaswth]

After moving some shared scripts to using gobin for several project tools, a colleague of mine expressed a concern that .gobincache directories in every project might eat up a lot of disk space due to having lots of duplicate binaries for the many projects he may be working on.

I'm not sure what the reasoning for having the -m binaries cached in a per-project fashion, so it would be useful to know what that is.

Maybe it would be possible to provide some option (perhaps an environment variable) to change the cache location?

[myitcv]

Thanks for the feedback @douglaswth

The honest answer is that it was the easiest thing to do in the first instance and I haven't yet gotten around to doing anything better 😄

The correct way to do this is to have all binaries in the same cache, keyed by the hash of the transitive dependencies (following the same pattern as the cmd/go build cache).

That said, things shouldn't be too bad as it stands: each of these separate caches is trimmed (i.e. old entries are purged) when the cache is updated. The only slightly annoying thing is that there is a cache per module, which needs to be git ignored (or equivalent).

Let me retitle this issue to be the placeholder for the proper fix.