ARROW-5044: [Release][Rust] Use stable toolchain for format check in …

…verification script

Author: Kouhei Sutou <kou@clear-code.com>

Closes #4059 from kou/release-fix-rust-format and squashes the following commits:

80af05f <Kouhei Sutou>  Use stable toolchain for format check in verification script

dev/release/verify-release-candidate.sh

@@ -320,14 +320,14 @@ test_rust() {
   # build and test rust
   pushd rust
 
+  # raises on any formatting errors
+  rustup component add rustfmt --toolchain stable
+  cargo +stable fmt --all -- --check
+
   # we are targeting Rust nightly for releases
   rustup default nightly
 
-  # raises on any formatting errors
-  rustup component add rustfmt-preview
-  cargo fmt --all -- --check
   # raises on any warnings
-
   RUSTFLAGS="-D warnings" cargo build
   cargo test