plugin-postcss has uninstalled dependency

[yurylavrukhin]

Hi! I'm using snowpack plugin-postcss.

When I run snowpack install — I get this error:
Error: @snowpack/plugin-postcss tried to access execa, but it isn't declared in its dependencies; this makes the require call ambiguous and unsound.

I think the problem is that this package uses uninstalled dependency execa.

This PR should fix the issue. Please check it out.

I'm using yarn2 and snowpack.