Mk/Uses/shebangfix.mk: Fix SHEBANG_FILES to use find(1)

Ports like net/vmware-vsphere-cli use SHEBANG_FILES with globs like so:

SHEBANG_FILES= bin/* ...

As of FreeBSD 11.1-RELEASE sed has changed and errors if attempted on non-file
objects. In the case of the cited port there are many other files in the
bin/ directory which are symlinks for compatibility with old scripts.
This causes the port patching to fail.

PR: 221229
Differential Revision: https://reviews.freebsd.org/D11853