cmd-build-with-buildah: selectively source NAME from argsfile

In [1] we added `DESCRIPTION=Fedora CoreOS testing-devel` to
build-args.conf, but this isn't exactly proper bash syntax.
Bash would want `Fedora CoreOS testing-devel` to be in quotes,
but `podman`/`docker` don't really support that in env file [2] [3]
and I assume args file is the same. If I added quotes there I get
"org.opencontainers.image.description": "\"Fedora CoreOS branched\"".

[1] coreos/fedora-coreos-config@90109c4
[2] containers/podman#9446
[3] docker/for-linux#1208

Author: dustymabe

src/cmd-build-with-buildah

@@ -179,8 +179,7 @@ build_with_buildah() {
         set -- "$@" --layers=true
         # output to a tag since it's more convenient for development;
         # buildah doesn't support doing both at once
-        # shellcheck disable=SC1090
-        osname=$(source "src/config/${argsfile}"; echo "${NAME}")
+        osname=$(eval "$(grep 'NAME=' "src/config/${argsfile}")"; echo "${NAME}")
         final_ref="containers-storage:localhost/${osname}:${VERSION}"
     else
         # In the supermin path ensure the ociarchive gets compressed