fix: allow setting PROTOC environment variable

[MatisseB]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• [ ] N
• [x] Y

fix #1413

2. What is the scope of this PR (e.g. component or file name):

kclvm/third-party/prost-wkt/wkt-types/build.rs kclvm/api/build.rs

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

• [ ] Affects user behaviors
• [ ] Contains syntax changes
• [ ] Contains variable changes
• [ ] Contains experimental features
• [ ] Performance regression: Consumes more CPU
• [ ] Performance regression: Consumes more Memory
• [x] Other

This fix ensures that if the PROTOC environment variable is already set, it will be used; otherwise, it defaults to using the protoc_bin_vendored binaries. This improves the flexibility of the protocol buffer compiler setup.

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• [x] N
• [ ] Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• [ ] Unit test
• [ ] Integration test
• [ ] Benchmark (add benchmark stats below)
• [ ] Manual test (add detailed scripts or steps below)
• [x] Other

No tests seams to be required for this change as the integration is pretty straightforward.

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