Use bin/yarn consistently for installations

Use bin/yarn consistently for installations

Since rails 7, the `rails yarn:install` command no longer uses
bin/yarn and so a) doesn't detect yarn binaries named "yarnpkg"
(e.g. Ubuntu) and b) doesn't allow us to pass any flags to yarn,
(e.g. "--ignore-engines").

Since we want to avoid making the installation more difficult (by
e.g. installing yarn from other sources, or requiring particular
node versions) we can instead just use our existing bin/yarn script.

The reason Dockerfile did its own thing was to avoid pulling in
big chunks of rails, but bin/yarn is a standalone ruby script.

Fixes #3579