Fix subtle build breakage

Repro case:
$ make clean generated_files
$ hack/update-generated-protobuf.sh

This would complain about not finding `fmt`, and it was indicating the wrong
GOROOT.  The problem was that the first step built binaries for generating
code, which *embeds* the value of GOROOT into the binary.  The whole tree was
bind-mounted into the build container and then JUST the dockerized dir was
mounted over it.  The in-container build tried to use the existing binaries,
but GOROOT is wrong.

This change whites-out the whole _output dir.

I first made just an anonymous volume for _output, but docker makes that as
root, which means I can't write to it from our non-root build.  So I just put
it in the data container.  This seems to work.  The biggest change this makes
is that the $GOPATH/bin/ and $GOPATH/pkg/ dirs will persist across dockerized
builds.

hack/update-generated-protobuf-dockerized.sh

@@ -45,7 +45,7 @@ gotoprotobuf=$(kube::util::find-binary "go-to-protobuf")
 # searches for the protoc-gen-gogo extension in the output directory
 # satisfies import of github.com/gogo/protobuf/gogoproto/gogo.proto and the
 # core Google protobuf types
-PATH="${KUBE_ROOT}/_output/local/go/bin:${PATH}" \
+PATH="${KUBE_ROOT}/_output/bin:${PATH}" \
   "${gotoprotobuf}" \
   --proto-import="${KUBE_ROOT}/vendor" \
   --proto-import="${KUBE_ROOT}/third_party/protobuf" \