Shorten the cargo integration test directory name

Right now the win64 snapshot builders are failing to produce a snapshot, and
I've managed to track it down to a path length issue. Windows paths have a
maximum of 260 characters, and the characters add up pretty fast for a path
like:

    c:\bot\slave\cargo-nightly-win-64
      \build\target\x86_64-w64-mingw32
      \cargo-integration-test\home\.cargo
      \git\checkouts\meta-dep-<hash>\
      \$SHA\.git\...

The normal builders aren't failing I presume because `cargo-nightly-win-64` is
longer than `cargo-win64-64` (we must be *right up* against the limit). I've
confirmed that this shortening fixes the tests on the bots.