eclass-overlay: Add pluginvm to cups-proxy group

The Plugin VM dispatcher needs access to the CUPS proxy socket,
so let's add its user (pluginvm) to the cups proxy group.

BUG=b:117218264
TEST=Build and boot

Cq-Depend: chromium:1820011
Change-Id: I52cfe1179b76f75c8d689f631584abef93608c75
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/1820182
Reviewed-by: Allen Webb <allenwebb@google.com>
Reviewed-by: Sean Kau <skau@chromium.org>
Commit-Queue: Dmitry Torokhov <dtor@chromium.org>
Tested-by: Dmitry Torokhov <dtor@chromium.org>
diff --git a/profiles/base/accounts/group/cups-proxy b/profiles/base/accounts/group/cups-proxy
index bf98bc5..2f938a9 100644
--- a/profiles/base/accounts/group/cups-proxy
+++ b/profiles/base/accounts/group/cups-proxy
@@ -1,4 +1,5 @@
 group:cups-proxy
 gid:20136
-# VMs run as crosvm user
-users:crosvm,cups-proxy
+# VMs run as crosvm user, also Plugin VM dispatcher needs to know list
+# of available printers to pass to the plugin.
+users:crosvm,cups-proxy,pluginvm