chore: fix scripts/bin/yarnpkg symlink

[paul-soporan]

What's the problem this PR addresses?

The scripts/bin/yarnpkg symlink has been broken for some time due to Windows weirdness:

➜  berry git:(master) readlink scripts/bin/yarnpkg             
#!/usr/bin/env node

require(`${__dirname}/../../packages/yarnpkg-cli/sources/boot-cli-dev.js`);

How did you fix it?

Changed it from a symlink to a regular file (a copy of scripts/bin/yarn) so that it doesn't cause issues on windows.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.