Makefile: pass cargo --workspace, (nearly) always

Makefile: pass cargo --workspace, (nearly) always

This seems to work around
  https://github.com/rust-lang/cargo/issues/9114
  https://github.com/rust-lang/cargo/issues/9114

The WASM call doesn't have it because (i) we want that to be minimal
(ii) there's only the one call to cargo for that architecture.

Build times:

  touch src/updates.rs; (time make -j12 wdt) 2>&1 |ts -s %.s
    before 26s    after 19s

  touch src/updates.rs; (time make -j12) 2>&1 |ts -s %.s
    before 33s    after 27s

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>