set the TARGET envar for the stdsimd integration test

[scampi]

The stdsimd crate fails to compile with the following. Setting the TARGET envar fixes the problem.

error: custom attribute panicked
   --> crates/core_arch/src/x86/fxsr.rs:102:5
    |
102 |     #[simd_test(enable = "fxsr")]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: message: TARGET environment variable should be set for rustc: NotPresent

[scampi]

the crate futures-rs fails with rust-lang/futures-rs#1453

[topecongiro]

Thanks!