use npm update instead of npm install on ~/.npm/_npx-globals

[zkat]

Right now, if you don't have a certain package in $PATH, we always do a npm install --global into $(npm config get cache)/_npx. This means that we'll run through an actual install cycle every time, instead of just making sure we're at the latest version.

So, we should use npm update instead, which should short-circuit this.

Unfortunately, npm is kinda wonky with npm update and globals right now, so we need to fix up npm update for this use case before we can take advantage of it.

[zkat]

I'm gonna close this for now, because speeding up installs is going to be a bit more involved after some recent changes meant to isolate npx runs from each other. This should still be optimized, but I don't think it's the only (or best) way.