Remove env hack for invalidating `run` without invalidating mount cache

[coryb]

A hack was introduced in #147 to add an environment variable to the llb.ExecOp when ignoreCache is used instead of using llb.IgnoreCache. The llb option has the undesired effect of also invalidating all cache associated with mounts, which is generallly not what users want in HLB.

There is an open issue in buildkit: https://github.com/moby/buildkit/issues/1213

We should re-evaluate and remove this hack when the above issue is resolved.