Special configs for Clippy

[justsmth]

Description of changes:

• Enable/disable lints depending on whether clippy is being used.

Call-outs:

• With this change, Clippy can only be run using the nightly toolchain. Using the stable toolchain produces the following error:

error[E0554]: `#![feature]` may not be used on the stable release channel
 --> aws-lc-sys/builder/main.rs:6:21
  |
6 | #![cfg_attr(clippy, feature(custom_inner_attributes))]
  |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.52%. Comparing base (c358484) to head (7eb3b47).
Report is 84 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #542      +/-   ##
==========================================
- Coverage   95.80%   92.52%   -3.28%     
==========================================
  Files          61       67       +6     
  Lines        8143     9277    +1134     
  Branches        0     9277    +9277     
==========================================
+ Hits         7801     8584     +783     
- Misses        342      422      +80     
- Partials        0      271     +271     

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