Bump xbuild to 0.6.0 to fix "build" command

[MaciejBaj]

Bump cargo-xbuild to 0.6.0 version to fix the build command which fails on the recent cargo version with error:

[1/3] Building cargo project
ERROR: Building with xbuild

Caused by:
    `rust-src` component not found. Run `rustup component add rust-src`.

As described on cargo-xbuild repo, version 0.6.0 fixes issue with rust-src component not found in :

Note: The latest version of cargo-xbuild supports all nightlies after 2020-07-30. If you are on an older nightly, you need to install version 0.5.35: cargo install cargo-xbuild --version 0.5.35.

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[ascjones]

Thanks! Closing though since just merged duplicate in #71.