pkg/invoke: ensure that custom env vars are prepended to the env

[ericchiang]

Ensure that custom values take priority over values defined in the
environment by pre-pending them to the env, not appending.

closes #410

[squeed]

LGTM.

[cmluciano]

lgtm

[ericchiang]

bump. what's the process for merging?

[rosenhouse]

The process for merging is described in our Acceptance policy.

I've added a unit test to cover this change. I think it's good to go.

[rosenhouse]

From the release notes for Go 1.9 beta:

The os/exec package now prevents child processes from being created with any duplicate environment variables. If Cmd.Env contains duplicate environment keys, only the last value in the slice for each duplicate key is used.

That would seem to break the intended behavior of this PR.

[bboreham]

Ha ha, don'tcha love it when two groups fix a problem in conflicting ways...

[rosenhouse]

Just hit this. Had run export CNI_COMMAND=VERSION in my shell and then tried to run some tests that used invoke. Fun times debugging.

Once we figure out how to get #538 green and merge, we should flip this back to appending.