Task does not resolve BUILD_ARG_* params correctly

[kardashov]

How to reproduce:

• Take this Dockerfile https://github.com/concourse/oci-build-task/blob/master/testdata/build-args/Dockerfile
• configure task with following params:

params:
  BUILD_ARG_some_arg: some_value  
  BUILD_ARG_some_other_arg: some_other_value

Build fails with error:
error: failed to solve: rpc error: code = Unknown desc = executor failed running [/bin/sh -c test "$some_arg" = "some_value "]: exit code: 1