Feature Request: Allow environment variable expansion context to be set for launch action

[cgrindel]

Related to #906.

Today, the environment variable expansion context defaults to the launch action target. It may be desirable to allow a client to set it to no context.

[brentleyjones]

I don't believe this should block 1.0.

[brentleyjones]

The new xcshemes API which is used with incremental generation mode supports this.