buildifier formatter should pass the path to the file being formatted

[malcolmr]

buildifier recently changed formatting behaviour depending on whether you are formatting a BUILD file or a .bzl file. For example, for BUILD it sorts all parameter lists by name.

codefmt invokes buildifier via stdin/stdout, so buildifier doesn't know whether it is a BUILD file or .bzl (and assumes the former, leading to spurious changes).

We can specify that path using the existing -path flag, even if we're formatting via a pipe.