[Rust] Pin to specific Rust nightly release

[asfimport]

Builds are currently non-deterministic because rust-toolchain contains "nightly" meaning "use the latest nightly release of Rust". This can cause build seemingly random build failure in CI. I propose we modify rust-toolchain to refer to a specific nightly release e.g. "nightly-2019-07-31" so that builds are deterministic.

We can update this nightly version when needed (e.g. to pick up new features) as part of the regular PR process.

Reporter: Andy Grove / @andygrove
Assignee: Andy Grove / @andygrove

PRs and other links:

• GitHub Pull Request #5324

_{Note: This issue was originally created as ARROW-6100. Please see the migration documentation for further details.}

[asfimport]

Neville Dipale / @nevi-me:
Issue resolved by pull request 5324
#5324