obs: build_binpkg --docker: set metavar to DISTRO

Make it obvious that the argument passed to --docker is the Linux
distribution that will be used.

$ ./build_binpkg.py -h
usage: build_binpkg.py [-h] [-d [DISTRO]] [-j JOBS] [-r] [-v] package

Change-Id: Ibf6f1a8fce7fd13795f1c25c75e14fb9eb8aa2b6
---
M scripts/obs/build_binpkg.py
1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/scripts/obs/build_binpkg.py b/scripts/obs/build_binpkg.py
index f647869..cda0193 100755
--- a/scripts/obs/build_binpkg.py
+++ b/scripts/obs/build_binpkg.py
@@ -31,7 +31,7 @@
                     " with build_srcpkg.py."
                     f" Output dir: {lib.config.path_temp}/binpkgs")
     parser.add_argument("-d", "--docker", type=arg_type_docker_distro,
-                        const=distro_default, nargs="?",
+                        const=distro_default, nargs="?", metavar="DISTRO",
                         help="build the package in docker for a specific"
                              f" distro (default: {distro_default}, other:"
                              f" almalinux:8, debian:10, ubuntu:22.04 etc.)")

