Swift Environment Variables setting is not used when running swift command plugins

[adam-fowler]

Describe the bug
The Swift Environment Variables setting is not used when running swift command plugins.

Given the extension uses the swift environment variables to set DEVELOPER_DIR it is probably a good idea we do include them in the environment while running swift command plugins

[rbenegal]

Verified with 9f874ecd using a CommandPlugin to check that the env variable is shown when run.