Include zsh completion with installer

Closes #6698.

PiperOrigin-RevId: 228890940
diff --git a/scripts/BUILD b/scripts/BUILD
index 95133bc..18e6943 100644
--- a/scripts/BUILD
+++ b/scripts/BUILD
@@ -42,6 +42,7 @@
         "//scripts/docs:srcs",
         "//scripts/packages:srcs",
         "//scripts/release:srcs",
+        "//scripts/zsh_completion:srcs",
     ],
     visibility = ["//:__pkg__"],
 )