Update CI config

[taiki-e]

• Update actions/checkout action to v4
• Reduce frequency of scheduled job (daily -> weekly)
  We use stable toolchain in most cases, so we usually don't need to check it every day.
• Remove no longer needed argument for cargo fmt
• Use cargo-hack's --rust-version flag for minrust job
  This respects rust-version field in Cargo.toml, so it removes the need to manage MSRV in both the CI file and Cargo.toml.