fix(config): add flatOptions.npxCache

[wraithgar]

This adds a new npxCache flatOption for libnpmexec to look for to put
its _npx content. libnpmexec will have to be patched to use that
value, and continue to pass flatOptions.cache to pacote et al.

The flatOptions cache is the one that is intended to be passed down
into other modules, as it has the _cacache suffix attached. What was
happening before was that npx was creating a new alternate cache one
directory up from where everything else was, and also putting the _npx
content there. It is possible this is the source of at least some of
our "npx doesn't find the right versions" bugs.

[wraithgar]

Test change fails before the fix was implemented and pass with it implemented.

[ruyadorno]

I know at least libnpmexec is expecting cache to be up a level: https://github.com/npm/libnpmexec/blob/5dcbb9a28200f13037a0190843337b0159533890/lib/cache-install-dir.js

let's make sure to fix it there first in case we want to land this one

[ruyadorno]

I mean, assuming the intent was to have npm exec placing things at ~/.npm/_npx instead of ~/.npm/_cacache/_npx - but that assumption might be wrong

[wraithgar]

libnpmexec expects it to be in BOTH places, because it passes it down into pacote which expects that value to be the "cacache" cache option:

https://github.com/npm/libnpmexec/blob/ac28b80f1a6e3b51fa19a155cb46dd5c57fe8cc0/lib/index.js#L106-L110

[ruyadorno]

libnpmexec expects it to be in BOTH places, because it passes it down into pacote which expects that value to be the "cacache" cache option:

https://github.com/npm/libnpmexec/blob/ac28b80f1a6e3b51fa19a155cb46dd5c57fe8cc0/lib/index.js#L106-L110

😱

[ruyadorno]

LGTM 👍