diagnostics: add cros_healthd user to the disk group

cros_healthd reports the size of non-removable block
devices on the system. To get this information, cros_healthd
needs to send ioctls to various /dev nodes, all of which
are only accessible by either root or the disk group.

BUG=b:128683357
TEST=ran build_packages and build_image for nami, deployed to
a DUT and checked grep disk /etc/group, made sure cros_healthd
was a member.

Cq-Depend:chromium:1757098
Change-Id: Ie8bd6d303a1c8568436f6a375a5144333dfc547c
Signed-off-by: Paul Moy <pmoy@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1757214
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
diff --git a/profiles/base/accounts/group/disk b/profiles/base/accounts/group/disk
index bce16ee..f425190 100644
--- a/profiles/base/accounts/group/disk
+++ b/profiles/base/accounts/group/disk
@@ -1,3 +1,3 @@
 group:disk
 gid:6
-users:root,adm,cros-disks
+users:root,adm,cros-disks,cros_healthd