sys-apps/yarn: Set installationMethod = portage

sys-apps/yarn: Set installationMethod = portage

By setting the installation method, yarn will do one of two things:

1) For older versions with no message for installationMethod = portage*,
no message will be shown. (without this, a message would be shown
recommending the user download and install a new version using a shell
script)

2) For versions that understand installationMethod = portage*,
a message telling the user to update yarn via portage will be shown.

*: https://github.com/yarnpkg/yarn/pull/7123

Closes: https://github.com/gentoo/gentoo/pull/11395
Package-Manager: Portage-2.3.62, Repoman-2.3.11
Signed-off-by: Jonathan Janzen <jjjonjanzen@gmail.com>
Signed-off-by: Zac Medico <zmedico@gentoo.org>