Disallow external bindgen when AWS_LC_SYS_EXTERNAL_BINDGEN=0

[justsmth]

Description of changes:

Disallow external bindgen usage when AWS_LC_SYS_EXTERNAL_BINDGEN=0 is set in the environment.

• This allows us to verify that the pregenerated bindings are being used.

Testing

• s390x-unknown-linux-gnu fails when this is set: https://github.com/aws/aws-lc-rs/actions/runs/14978763694/job/42077580194?pr=808#step:10:1085

...
   --- stderr
  Failure invoking external bindgen! external bindgen usage prevented by AWS_LC_SYS_EXTERNAL_BINDGEN=0

  thread 'main' panicked at aws-lc-sys/builder/main.rs:753:5:
  aws-lc-sys build failed. Please enable the 'bindgen' feature on aws-lc-rs or aws-lc-sys.For more information, see the aws-lc-rs User Guide: https://aws.github.io/aws-lc-rs/index.html
...

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 92.34%. Comparing base (c358484) to head (c49a7ef).
Report is 218 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #808      +/-   ##
==========================================
- Coverage   95.80%   92.34%   -3.46%     
==========================================
  Files          61       71      +10     
  Lines        8143     8972     +829     
  Branches        0     8972    +8972     
==========================================
+ Hits         7801     8285     +484     
- Misses        342      418      +76     
- Partials        0      269     +269     

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