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/lib/golang.sh

@@ -612,18 +612,18 @@ kube::golang::build_binaries() {
     local use_go_build
     local -a targets=()
     local arg
-    
+
     # Add any files with those //generate annotations in the array below.
     readonly BINDATAS=( "${KUBE_ROOT}/test/e2e/framework/gobindata_util.go" )
     kube::log::status "Generating bindata:" "${BINDATAS[@]}"
     for bindata in ${BINDATAS[@]}; do
-	  # Only try to generate bindata if the file exists, since in some cases
-	  # one-off builds of individual directories may exclude some files.
+          # Only try to generate bindata if the file exists, since in some cases
+          # one-off builds of individual directories may exclude some files.
       if [[ -f $bindata ]]; then
           go generate "${bindata}"
       fi
     done
-    
+
     for arg; do
       if [[ "${arg}" == "--use_go_build" ]]; then
         use_go_build=true