chore(internal/kokoro): create tmp dir, not file (#657)

diff --git a/internal/kokoro/synth/synth.sh b/internal/kokoro/synth/synth.sh
index 28f9699..e223e3d 100755
--- a/internal/kokoro/synth/synth.sh
+++ b/internal/kokoro/synth/synth.sh
@@ -9,7 +9,7 @@
 cd $(dirname $0)/../../..
 
 # Install Go 1.15
-tempdir=$(mktemp)
+tempdir=$(mktemp -d)
 curl -o /tmp/go.tgz https://dl.google.com/go/go1.15.1.linux-amd64.tar.gz &&
     tar -C $tempdir -xzf /tmp/go.tgz &&
     rm /tmp/go.tgz &&