buildah --os xor --arch still warns about missing TARGETOS/TARGETARCH

[edsantiago]

Followup to #4295.

$ printf "FROM alpine\nARG TARGETOS\nENV a=b\n" | bin/buildah build --os linux -q -
WARN[0000] missing "TARGETOS" build argument. Try adding "--build-arg TARGETOS=<VALUE>" to the command line 
da4bb4edfe98d2b868c91433bad7c082e9cc8b4070c05fc0107a64e040515f7b

Likewise with --arch and TARGETARCH only. Specifying both on the command line silences the warnings. (As does --platform, as fixed in #4295).