rustpkg should use workcache

[catamorphism]

Sub-bug of #5677

rustpkg build should "work like make": it should only rebuild files whose dependencies have changed. (Currently, it rebuilds everything every time.) Doing this will involve using workcache, which has its own set of issues (see #4432).

[huonw]

Visiting for triage; some improvements to workcache landed in #7885 (but #4432 is still open, and there are still a pile of FIXMEs in it).

[catamorphism]

#9034 will close this.

[catamorphism]

#9034 closed this. If other issues arise, file a more specific bug.