Re-link bootstrap.el whenever straight.el is rebuilt

[raxod502]

Currently, if you change the :local-repo for straight.el, the bootstrap script is not re-linked, and you have to make a network call using install.el. This is undesirable.

(Of course, we'll always have the edge case of making a copy of straight.el under a different :local-repo and switching to it, but not reloading your init-file before restarting. This should be documented in the developer manual, see #51.)

[raxod502]

I've decided that it makes more sense to not use the symlink and instead just require users who customize :local-repo for straight.el (probably 0 users) to change their bootstrap snippet to point at the appropriate local repository. I am implementing this change.