Support for passing through raw buildkit policies

This adds an env var which can be used to pass in a path to a file to
read a buildkit source poliy from.

This is applied to any build is executed with the env set.
It is also applied to bakes (which are calling build behind the scenes).

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

bake/bake.go

@@ -1145,6 +1145,11 @@ func toBuildOpt(t *Target, inp *Input) (*build.Options, error) {
 	}
 	bo.Attests = controllerapi.CreateAttestations(attests)
 
+	bo.SourcePolicy, err = build.ReadSourcePolicy()
+	if err != nil {
+		return nil, err
+	}
+
 	return bo, nil
 }