Temporarily pin toolchain version to avoid clippy #400

Job	Run time
cargo test (win64)	1h 13m 29s
Check License Header	2m 22s
Check cargo fmt	34s
cargo check	12m 44s
build with wasm-pack	2m 55s
cargo test (macos)	12m 36s
cargo test (macos-aarch64)	11m 58s
Verify MSRV (Min Supported Rust Version)	9m 34s
Verify Vendored Code	49s
cargo test (amd64)	12m 14s
check configs.md and ***_functions.md is up-to-date	3m 8s
check Cargo.toml formatting	2m 52s
clippy	6m 49s
cargo test hash collisions (amd64)	28m 19s
cargo doc	4m 37s
cargo test datafusion-cli (amd64)	4m 11s
cargo test doc (amd64)	16m 49s
cargo examples (amd64)	14m 11s
Run sqllogictest with Postgres runner	4m 4s
cargo test pyarrow (amd64)	3m 2s
verify benchmark results (amd64)	13m 0s
	4h 0m 17s

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