fix(checkYarn): Use realpath to detect corepack

[Tobbe]

rw exits with a non-descript error when checking the yarn installation

Even the debug log is pretty unhelpful

Just says command failed with error and exit status 1.
No clues as to what actually caused it to fail

With this PR you'd get a more helpful log message:

No project found in /Users/tobbe/dev/redwood/rw-cli
yarn exec <commandName> ...

And to get around the actual error I implemented a call to realpath $(which yarn). If that check passes I skip yarn exec env. If the realpath check fails the code still falls back to yarn exec env.

The usage of yarn exec env comes from here https://yarnpkg.com/corepack#installation
But it apparently only works if you're already in an initialized yarn project.

The "official" recommended real solution to this seems to be to enable corepack in a temp directory and use that.
nodejs/corepack#113

But before going down that route I just did the realpath check for now as that was much easier and faster to implement.

I also asked on the yarn discord about this. See if they come back with any other ideas before we implement anything too crazy here. https://discord.com/channels/226791405589233664/654372321225605128/1205773363981848586