selftests: rds: Add SUDO_USER env variable

selftests: rds: Add SUDO_USER env variable

This patch modifies rds selftests to use the environment variable
SUDO_USER for tcpdumps if it is set.  This is needed to avoid chown
operations on the vng 9pfs which is not supported.  Passing a user
listed in sudoers avoids the tcpdump privilege drop which may
otherwise create empty pcaps

Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260504054143.4027538-7-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>