Add command field to BuildStarted in build-analysis

[kurasaiteja]

What does this PR try to resolve?

This PR adds a command field to LogMessage::BuildStarted that records the CLI arguments Cargo was invoked with. Currently, when reviewing build-analysis logs in ~/.cargo/log/, there's no way to tell which command triggered a particular build session.
Added capture std::env::args() and included it in the BuildStarted message.

Example Output:

  {
    "reason": "build-started",
    "command": ["cargo", "build", "--release", "-p", "foo"],
    "cwd": "/home/user/project",
    ...
  } 

How to test and review this PR?

Run the build-analysis tests using cargo test --test testsuite build_analysis
All 10 tests pass, including log_msg_build_started which verifies the new field is present.

To manually test:

export CARGO_BUILD_ANALYSIS_ENABLED=true
cargo +nightly build -Zbuild-analysis
cat ~/.cargo/log/*.jsonl | head -1 | jq .command

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r? @weihanglo

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[weihanglo]

Thanks!

I may probably collect args_os and do lossy conversion, so that at least we won't panic if there is any non-UTF8 arg

[kurasaiteja]

Good one! Will change

[weihanglo]

Thanks!

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