selftests: net: use BASH for bareudp testing

selftests: net: use BASH for bareudp testing

In bareudp.sh, this script uses /bin/sh and it will load another lib.sh
BASH script at the very beginning.

But on some operating systems like Ubuntu, /bin/sh is actually pointed to
DASH, thus it will try to run BASH commands with DASH and consequently
leads to syntax issues:
  # ./bareudp.sh: 4: ./lib.sh: Bad substitution
  # ./bareudp.sh: 5: ./lib.sh: source: not found
  # ./bareudp.sh: 24: ./lib.sh: Syntax error: "(" unexpected

Fix this by explicitly using BASH for bareudp.sh. This fixes test
execution failures on systems where /bin/sh is not BASH.

Reported-by: Edoardo Canepa <edoardo.canepa@canonical.com>
Link: https://bugs.launchpad.net/bugs/2129812
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://patch.msgid.link/20251027095710.2036108-2-po-hsu.lin@canonical.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>