Makefile: Only run `xcodebuild` when there are changes

Previously, Make would always run `xcodebuild`. I couldn't figure out
why, but after some looking around and experimenting, it looked to be
due to the `$(RUST_LIB)` prerequisite. Putting it in the order-only
dependencies list seems to fix the never-up-to-date problem.

Also use `:=` instead of `=` when defining `SOURCE_FILES`. During my
research, I came across this recommendation:

> always use := not = for shell (and wildcard, for that matter) for
> performance reasons.

From MadScientist (https://stackoverflow.com/users/939557/madscientist)
on Stack Overflow:
https://stackoverflow.com/questions/26694249/makefiles-using-wildcard-vs-find-for-specifying-source-files/26694693#26694693

Move the `DomeKey` debug product executable path into a variable and use
it as a target to clarify the build target and also allow us to
substitute the variable in the `run` task.

Now finally, `make run` won't re-build the project if no changes have
been made, it'll just run the executable.

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