cargo: Do builds under ./.cargo/work in local mode

Doing builds outside of the cwd is breaking rustc's logic for
locating crates in ./.cargo/lib.

Ideally, cargo would not be changing directories at all.

src/cargo/cargo.rs

@@ -661,7 +661,7 @@ fn configure(opts: options) -> cargo {
         installdir: p,
         bindir: path::connect(p, "bin"),
         libdir: path::connect(p, "lib"),
-        workdir: path::connect(home, "work"),
+        workdir: path::connect(p, "work"),
         sourcedir: path::connect(home, "sources"),
         sources: sources,
         mut current_install: "",