rustpkg: workcache causes problems for "rustpkg install"

[MicahChalmer]

There are some problems with workcache and rustpkg install, which I'm guessing stem from the same underlying problem:

1. If you do a successful rustpkg build, then a subsequent rustpkg install won't install anything (nothing is copied to bin, and bin is not created if it doesn't exist) but rustpkg will return a successful exit status, and print a few messages about files being fresh.

2. If you do a successful rustpkg install (one which actually installs--not one that exhibits the problem above) and then remove the binary from bin, a subsequent rustpkg install or rustpkg build will fail with something like this:

   task failed at 'Unhandled condition: bad_stat: (std::path::PosixPath{is_absolute: true, components: ["home", ~"micah", ~"progs", ~"rustpkg-wc-bug", ~"bin", ~"myapp"]}, ~"Couldn't get file access time")'

Removing .rust/rustpkg_db.json from the workspace fixes both problems (temporarily, until the next rustpkg build or install puts it back.)

[catamorphism]

Yes, I see why this is true. Good catch, I'll look into it.

[catamorphism]

I have a patch fixing this bug, but it's blocked on #9263 landing.