setup-apt-repo: Install groonga-keyring.

This allows the system to get updates to the Groonga repository
signing key, so `apt update` doesn’t start failing when the key
changes (like it recently did).

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

scripts/lib/setup-apt-repo

@@ -63,4 +63,8 @@ else
     apt-get update && rm -f "$STAMP_FILE"
 fi
 
+if [ "$release" = "stretch" ]; then
+    apt-get install -y groonga-keyring
+fi
+
 echo "$DEPENDENCIES_HASH" > "$DEPENDENCIES_HASH_FILE"