chore(internal/kokoro): fix var typo (#655)

diff --git a/internal/kokoro/synth/synth.sh b/internal/kokoro/synth/synth.sh
index 105e2b7..28f9699 100755
--- a/internal/kokoro/synth/synth.sh
+++ b/internal/kokoro/synth/synth.sh
@@ -11,7 +11,7 @@
 # Install Go 1.15
 tempdir=$(mktemp)
 curl -o /tmp/go.tgz https://dl.google.com/go/go1.15.1.linux-amd64.tar.gz &&
-    tar -C tempdir -xzf /tmp/go.tgz &&
+    tar -C $tempdir -xzf /tmp/go.tgz &&
     rm /tmp/go.tgz &&
     export PATH=$tempdir/go/bin:$PATH