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ARROW-3868: [Rust] Switch to nightly Rust for required build, stable is now allowed to fail

Author: Andy Grove <andygrove73@gmail.com>

Closes #3034 from andygrove/ARROW-3868 and squashes the following commits:

1690438 <Andy Grove> fix bash if format
9cc083e <Andy Grove> fail build if examples fail
a602c6e <Andy Grove> fix bug
b3be151 <Andy Grove> Address PR feedback
ef0f068 <Andy Grove> use nightly Rust in release process
4843b54 <Andy Grove> Use separate scripts for nightly vs stable Rust
a55d35b <Andy Grove> update windows CI
8a6f938 <Andy Grove> cargo fmt using rust nightly
df6e2f6 <Andy Grove> Switch to nightly Rust for required build, stable is now allowed to fail