There’s a common tool included in Hadoop that also has a `yarn` command,
which created issues for users who had Hadoop installed:
* #1257
* #1363

Yarn also installs the command under `yarnpkg` alias (added in
https://github.com/yarnpkg/yarn/commit/cefa9a368dd3df1a1f1ecd9e50de9c5fabefbc92)
so we can use `yarnpkg` instead of `yarn` to make it more reliable.

This has no effect on users who don't have Hadoop installed, but those
who have won't see errors from falsely detecting Hadoop Yarn as Yarn
the package manager, and they can now also install Yarn to make use of
our Yarn support without the Hadoop Yarn interfering.