misc: update rust-toolchain file to TOML format

[adamqqqplay]

Relevant Issue (if applicable)

Dependabot can't parse our rust-toolchain. Because only rust-toolchain files formatted as TOML are supported in Dependabot. And the non-TOML format was deprecated by Rust.

Details

1. Move rust-toolchain to rust-toolchain.toml
2. Update the parsing process of rust-toolchain in the test script.

Types of changes

What types of changes does your PullRequest introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

Go over all the following points, and put an x in all the boxes that apply.

• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #1511 (7e70a22) into master (d793aee) will decrease coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1511      +/-   ##
==========================================
- Coverage   62.73%   62.73%   -0.01%     
==========================================
  Files         129      129              
  Lines       44158    44158              
  Branches    44158    44158              
==========================================
- Hits        27703    27702       -1     
  Misses      15088    15088              
- Partials     1367     1368       +1     

see 2 files with indirect coverage changes

[imeoer]

Any benefit to using the toml?

[adamqqqplay]

Any benefit to using the toml?

The main reason I submitted this PR was that Github Dependabot was unable to parse our legacy rust toolchain, which did not facilitate quick fixes for security vulnerabilities.

The rust-toolchain file in TOML format has been supported in rustup 1.23.0, bringing some new features. You can refer to: https://blog.rust-lang.org/2020/11/27/Rustup-1.23 .0.html#new-format-for-rust-toolchain