feat(servstate): expand environment variables for service commands

[zhijie-yang]

For reference, this fixes the command part of #182.

[cjdcordeiro]

fyi, the RK020 spec is not about this specifically.

[benhoyt]

Hi @zhijie-yang, thanks for the submission. Out of interest, do you have a need for this as part of the RK020 work, or is this just something you want to fix separately? The reason I ask is I'm pressed for time at the moment and may not get a chance to review this properly for a couple of weeks.

In the meantime, at a quick glance it looks a bit convoluted, and there's a fair bit of manual, error-prone parsing going on with strings.TrimLeft etc. It looks like it only addresses the command part of the #182 (my comment further down), not the main issue with expanding $VAR in the environment itself. Is that your intention? It would be good to see a proper PR description with some examples of what this covers.