Validate greenwave policy early in Commands.build
    
    Original method call to validate greenwave policy is too late and it is
    difficult to reuse _build in downstream tools. For example of fedpkg,
    _build is overridden to submit multiple builds in some case, which
    causes greenwave policy defined in gating.yaml will be validated
    multiple times accordingly.
    
    This change ensures the validation happens only once, whatever how _build
    is reused.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>