aarch64: Make the missing PLATFORM env var error message more useful

1 job for aarch64 in 3 minutes and 41 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #1052
img

00:03:41

 
Name Stage Failure
failed
img Test
info: syncing channel updates for 'nightly-2017-11-29-x86_64-unknown-linux-gnu'
info: latest update on 2017-11-29, rust version 1.24.0-nightly (73bca2b9f 2017-11-28)
info: downloading component 'rustc'
info: downloading component 'rust-std'
error: component download failed for rust-std-x86_64-unknown-linux-gnu
info: caused by: could not download file from 'https://static.rust-lang.org/dist/2017-11-29/rust-std-nightly-x86_64-unknown-linux-gnu.tar.xz' to '/root/.rustup/downloads/029b6244f16e107ce0539a8273420daf8ad5a4a80030f12de4621508bf84d8e3.partial'
info: caused by: error during download
info: caused by: [18] Transferred a partial file (transfer closed with 13272559 bytes remaining to read)
ERROR: Job failed: exit code 1