[sanitizer_common][test-only] Specify full path for sort executable in popen.cpp (#171622)

This test has begun failing on iossim with 'sh: sort: command not found'
in the stderr. I believe this may be due to the change to the lit
internal shell not having 'sort' in it's path.

This patch adds the full path /usr/bin/sort to work around this.

GitOrigin-RevId: c5995e25ba6a32b839cefe86abf721f57f9bf615