Rust BPF: Xargo does not use toolchain passes when building sysroot or dependent crates

[jackcmay]

Problem

Would expect the following to build the project, sysroot, and any dependent crates all with the bpf toolchain but it doesn't:

xargo +bpf build

In order to work around this either the default toolchain must be set to bpf:

rustup default bpf

Or local directory override must be set:

rustup override set bpf

Proposed Solution

Xargo should propagate the passed toolchain for everything it builds.

[jackcmay]

Already raised for xargo: japaric/xargo#123

Our workaround is to use the RUSTUP_TOOLCHAIN environment variable.