make WORKSPACE rules reproducible

per bazel error message:
DEBUG: Rule X indicated that a canonical reproducible form can be obtained by modifying arguments shallow_since = Y

Change-Id: I1c29609197d776536b7bc0336858047d7494d795
diff --git a/WORKSPACE b/WORKSPACE
index 36a26fe..65298b1 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -175,6 +175,7 @@
     name = "com_googlesource_gerrit_bazlets",
     remote = "https://gerrit.googlesource.com/bazlets",
     commit = "a511f3c90129d7de7ae67c0637001162980c08d5",
+    shallow_since = "1606931369 -0600",
 )
 
 load("@com_googlesource_gerrit_bazlets//:gerrit_api.bzl", "gerrit_api")
@@ -243,6 +244,7 @@
     name = "com_googlesource_gerrit_plugin_owners",
     remote = "https://gerrit.googlesource.com/plugins/owners/",
     commit = "17817c9e319073c03513f9d5177b6142b8fd567b",
+    shallow_since = "1593642470 +0200",
 )
 load("@com_googlesource_gerrit_plugin_owners//:external_plugin_deps_standalone.bzl", gerrit_owners_deps="external_plugin_deps_standalone")
 gerrit_owners_deps()