command: Make passed in env take higher priority

This MR fixes #3408 (closed) by adjusting the priority of envs for os.Environ() and passed in envs, and a test case is added to prove this.

By the way this MR also fixes test TestNewCommandProxyEnv, which seems intended to check proxy related envs can be whitelisted form os.Environ(), but instead passed in the envs through argument, making the test meaningless.