rustfmt and clippy not available for aarch64 since 1.36.0

[sameo]

aarch64 being a tier-2 Rust architecture, both rustfmt and clippy are not available for the 2 aarch64 toochains (musl and glibc) that we use, since 1.36.0.

Nightly (2019-12-16) has those 2 available for aarch64 though.

We just updated our CI container Dockerfile toolchain to 1.39.0, but since our buildkite default scripts run clippy on aarch64, we can not switch to the newly built container. Potential solutions:

1. Use a fixed nightly toolchain (2019-12-16 for example) for the aarch64 CI container until the next stable release supports clippy for that architecture.
2. Skip clippy check from our default buildkite pipeline

[sameo]

cc @andreeaflorescu @liujing2

[andreeaflorescu]

For rustfmt, we don't need to run it on both platforms. It will format all the code even if run only on x86_64. We can just remove the code from the pipeline. If that's the same for clippy, we can just remove clippy as well from the pipeline. What do you think?

[sameo]

AFAICS we already run cargo fmt only on x86_64, so that's fine.
I'm ok with removing clippy from the pipeline, at least for aarch64.

I'll proceed that way then.

[sameo]

See PR #23