Fix toolchain and CI, and update README

[zjp-CN]

This PR includes:

• stripping target triple from channel field (refer to https://rust-lang.github.io/rustup/overrides.html#channel )
• fixing a typo in component name
• updating actions/setup-python from v3 to v5
• updating README in usage with toolchain installation and override shorthand syntax

[zjp-CN]

Before: spurious build job

After: real building job ¹

Footnotes

1. the runner on Github Action has already contained libz3-4 and clang, so no need to install them. ↩