Specify rust toolchain explicitly, document how to change it #38665

Job	Run time
Check License Header	2m 21s
linux build test	5m 5s
Check cargo fmt	41s
build with wasm-pack	3m 31s
cargo test (macos-aarch64)	26m 5s
Verify MSRV (Min Supported Rust Version)	8m 10s
Verify Vendored Code	55s
cargo test (amd64)	16m 11s
cargo check functions	2m 18s
clippy	5m 53s
cargo check datafusion	5m 28s
cargo check no default features	8m 26s
cargo doc	4m 15s
cargo test doc (amd64)	17m 16s
cargo examples (amd64)	16m 39s
Run sqllogictest with Postgres runner	5m 23s
cargo test pyarrow (amd64)	3m 38s
verify benchmark results (amd64)	8m 56s
check Cargo.toml formatting	3m 13s
check configs.md and ***_functions.md is up-to-date	4m 19s
	2h 28m 43s

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