lxc-usernsexec: setgroups() similar to other places shouldn't fail on EPERM

lxc-usernsexec: setgroups() similar to other places shouldn't fail on EPERM

FAIL: lxc-tests: lxc-test-usernsexec (1s)
---
as test-userns executing /tmp/autopkgtest.waGEXj/build.Hm3/src/src/tests/lxc-test-usernsexec
uid=1001 gid=1001 name=test-userns subuid=165536 subgid=165536 ver=1:4.0.4-0ubuntu3
lxc-utils=1:4.0.4-0ubuntu3 kver=5.8.0-19-generic
USERNSEXEC=lxc-usernsexec
nouidgid: PASS
myuidgid: FAIL - runtest failed 1
  $ lxc-usernsexec -mu:0:1001:1 -mg:0:1001:1 -- /tmp/autopkgtest.waGEXj/build.Hm3/src/src/tests/lxc-test-usernsexec inside f0
  lxc 20200914222824.562 ERROR    utils - utils.c:lxc_setgroups:1363 - Operation not permitted - Failed to setgroups()
  kid 73112 is gone 1
subuidgid: PASS
bothsets: PASS
mismatch: PASS
ERRORS: myuidgid
---

Reported-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>