internal: pin staticcheck to 1.9

Change-Id: I2e09c7c31986a6463c0ecf60e1f1f754ae86a8a9
Reviewed-on: https://code-review.googlesource.com/c/37230
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Eno Compton <enocom@google.com>
diff --git a/internal/kokoro/vet.sh b/internal/kokoro/vet.sh
index 0b3e4ac..c5e90d8 100755
--- a/internal/kokoro/vet.sh
+++ b/internal/kokoro/vet.sh
@@ -66,7 +66,7 @@
 
 # TODO(deklerk) It doesn't seem like it, but is it possible to glob both before
 # and after the colon? Then we could do *go-cloud-debug-agent*:*
-staticcheck -ignore '
+staticcheck -go 1.9 -ignore '
 *:S1007
 *:SA1019
 cloud.google.com/go/firestore/internal/doc-snippets.go:*