Fix musl compilation and re-enable glibc screenshots

[hulto]

This change ensures that imix compiles successfully on musl static targets while re-enabling screenshot capabilities on glibc-based Linux systems. By isolating the xcap dependency behind a cfg(not(target_env = "musl")) flag in both Cargo.toml and the screenshot module, we handle unsupported library components selectively. A mock error message is provided for musl targets to maintain function signatures without breaking compilation.

---

PR created automatically by Jules for task 13380838908477848957 started by @hulto

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.

[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
660    ±0	660    ±0	0    ±0	0    ±0	0    ±0	0    ±0	1ms    ±0

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#1627	✅	660	660	0	0	0	0	39.3s

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
660	0	0	5.6s

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
TestDockerExecutor_Build_ContextCancellation	1	5.6s	5.6s
TestInteractiveShell	1	5.0s	5.0s
TestOtherStreamOutput	1	5.0s	5.0s
TestDockerExecutor_Build_SimpleEcho	1	3.0s	3.0s
TestDockerExecutor_Build_StreamsOutputInRealTime	1	1.4s	1.4s
TestGetRandomName/NoDuplicates	1	1.4s	1.4s
TestGetRandomName	1	1.4s	1.4s
TestDockerExecutor_Build_MultiLineOutput	1	1.1s	1.1s
TestCreateTestData	1	1.1s	1.1s
TestNonInteractiveShell	1	1.0s	1.0s

_{🎉 No failed tests in this run. | 🍂 No flaky tests in this run.}

Github Test Reporter by CTRF 💚

🔄 This comment has been updated

[hulto]

fails to connect to tcp port - bad.