Remove use of `-Zbuild-std` if and when the UEFI targets become tier-2

[nicholasbishop]

As mentioned in #502 (comment), we should remember to remove use of -Zbuild-std when official builds of the UEFI targets become available.

The -Zbuild-std flags can be removed from xtask/src/cargo.rs at that point, and we can also drop rust-src from the list of components installed by rust-toolchain.toml.

[nicholasbishop]

From #502 (comment):

We should also add a targets field in rust-toolchain.toml listing all the required targets. (This isn't very well documented in https://rust-lang.github.io/rustup/overrides.html as yet. Note that this is different from .cargo/config.toml's target field.)