Makefile: Do not rebuild the sidecar if not the default path

[martinetd]

(This is a follow-up on #56 -- it contains #56 itself as that'd conflict so leaving it as draft fo now)

If the SIDECAR variable has been overriden then running cargo is not
useful nor desired:
define a DEFAULT_SIDECAR variable we can compare against and only add
the $(SIDECAR):: rebuild rule if SIDECAR has the default value.

---

The last commit is complete garbage, but I couldn't come up with a way to flag that I don't want to rebuild the sidecar (the :: make target will always try to run cargo and that'll fail for me); I think I'll just patch it on the nixos side but bringing it up here if you have a better idea.

yeah, this is not good for "normal" build path. What we can do, perhaps, is to have DEFAULT_SIDECAR vs SIDECAR that can be overridden, and if they don't match, don't even trigger cargo (as it's pointless anyways). Let's do this as a separate PR still?

That makes a lot of sense, with the path changed it's not like cargo would be able to do anything useful anyway.

I've fiddled with it a bit and it appears to work with both default and non-default value in my environment.