skip hack/tools/vendor folder

The makefiles scripts create a variable with all the go files
that are part of the Kubernetes source tree, including staging.

As today, this variable has a size of < 100kb

wc .make/all_go_dirs.mk
2326  2326 98905 .make/all_go_dirs.mk

This variable is passed as argument in the Makefiles, where it
is expanded. In Linux, there is a limit to the max size of
the arguments MAX_ARG_STRLEN.

If the arguments go above 128k, you get a nice:

execvp: /usr/bin/env: Argument list too long

If you, for whatever reason, do some go mod vendor inside the
hack/tools folder, these files will be added to the variable
and most probably you'll go above the limit and get that error.

Then, you'll learn a lot about Makefils, shell expansion, strace,
execpve, ARG_MAX and MAX_ARG_STRLEN,until you realize what is
the real problem :).

hack/make-rules/helpers/cache_go_dirs.sh

@@ -39,14 +39,17 @@ function kfind() {
     # We want to include the "special" vendor directories which are actually
     # part of the Kubernetes source tree (./staging/*) but we need them to be
     # named as their ./vendor/* equivalents.  Also, we  do not want all of
-    # ./vendor or even all of ./vendor/k8s.io.
+    # ./vendor , ./hack/tools/vendor or even all of ./vendor/k8s.io.
     find -H .                      \
         \(                         \
         -not \(                    \
             \(                     \
                 -name '_*' -o      \
                 -name '.[^.]*' -o  \
-                -path './vendor'   \
+                \(                 \
+                  -name 'vendor'   \
+                  -type d          \
+                \)                 \
             \) -prune              \
         \)                         \
         \)                         \