Run now from this repo's working dir, not via npx

[shawnbot]

Currently we run the Now CLI with npx now <args>, which presents some issues:

1. Each repo is responsible for having now as a (dev) dependency, and may use a version that are "incompatible" with the assumptions we make here.
2. As seen in this failed run, if npx has to install now at runtime it'll fail because it's running as root, which prevents lifecycle hooks like postinstall from running, and it looks to me from that error that now relies on those.

So instead of running npx now, we should have now as a dependency in this repo and get its CLI path locally, i.e. ./node_modules/.bin/now.

The only downside to this is that repos can't spec their own Now CLI version, but being able to upgrade it consistently across our repos by pinning a version of this action feels more sustainable to me.