What?

Fix error with Netlify not finding pnpm create-next-app build output (when users use pnpm)

Also, remove unnecessary configuration, since pnpm v9 and v10 docs mention that the root package is always included:

The root package is always included, even when custom location wildcards are used.

• v9 docs https://pnpm.io/9.x/pnpm-workspace_yaml#:~:text=The%20root%20package%20is%20always%20included%2C%20even%20when%20custom%20location%20wildcards%20are%20used.
• v10 docs https://pnpm.io/pnpm-workspace_yaml#:~:text=The%20root%20package%20is%20always%20included%2C%20even%20when%20custom%20location%20wildcards%20are%20used.

For pnpm v9 (eg. latest version 9.15.9), the packages: [] cannot be entirely removed, because pnpm will error out with the message packages field missing or empty:

$ pnpm i
 ERROR  packages field missing or empty
For help, run: pnpm help install

Why?

When packages is set to a single item with ., Netlify infers the build command as pnpm --filter <package name from root package.json> build

$ pnpm --filter repro-pkg-name build
No projects matched the filters in "/Users/k/p/repro"
$ echo $?
0

How?

Change packages config from single item with . to an empty array, which causes Netlify to infer pnpm build as the command

cc @eps1lon