exp/vulncheck: make checks.bash work on Darwin

Fixes golang/go#51664

Change-Id: I03235cad0de24f825de2810d8f70336b12901d08
Reviewed-on: https://go-review.googlesource.com/c/exp/+/392594
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
diff --git a/vulncheck/checks.bash b/vulncheck/checks.bash
index a083efa..770dc28 100755
--- a/vulncheck/checks.bash
+++ b/vulncheck/checks.bash
@@ -34,11 +34,8 @@
   runcmd misspell -error .
 }
 
-# Support ** in globs for finding files throughout the tree.
-shopt -s globstar
-
 runchecks() {
-  check_header **/*.go **/*.bash
+  check_header *.go internal/*/*.go *.bash
   runcmd go vet -all ./...
   check_staticcheck
   check_unparam