prost-build doesn't generate rustfmt-safe code, so we had to add it to
the ignore list for rustfmt on "make check". However, the ignore list
isn't supported by stable rustfmt, so we have to work around this some
other way -- in this case, just do "rustfmt" on the generated file in
"build.rs" (this way, formatting errors in checked-in source code are
still caught but generated code doesn't cause spurrious errors).
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
