Fix Homebrew container install user

[eleanorjboyd]

Summary

Run the Homebrew setup commands in the homebrew/brew container as the linuxbrew user instead of root, while discovering the brew executable from the container PATH at runtime.

The container job still runs as root so GitHub Actions checkout/tooling can write to the mounted workspace, but recent homebrew/brew images have been brittle when brew install itself runs as root. PR #466 is currently failing before Rust setup/tests with Homebrew crashing while pouring the ca-certificates bottle:

undefined method '[]' for nil
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/utils/bottles.rb:127:in 'Utils::Bottles.load_tab'

This PR keeps the existing container shape but switches the install step to:

brew_bin="$(command -v brew)"
su linuxbrew -c "${brew_bin} update || true"
su linuxbrew -c "${brew_bin} install python@3.12 python@3.11"

That avoids hardcoding the Linuxbrew prefix while still avoiding root-owned Homebrew install behavior.

Validation

This is a CI-only workaround. The useful validation is the ci-homebrew-container Actions job on this PR.

[github-actions]

Performance Report (macOS)

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta
Server Startup	104ms	711ms	93ms	11ms
Full Refresh	197ms	37670ms	179ms	18ms

Results based on 10 iterations. P50 = median, P95 = 95th percentile.

---

Legend

• 🚀 Significant speedup (>100ms faster)
• ✅ Faster than baseline
• ➖ No significant change
• 🔺 Slower than baseline (>100ms)
• ⚠️ Significant slowdown (>500ms)

[github-actions]

Performance Report (Linux) ➖

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta	Change
Server Startup	1ms	1ms	1ms	0ms	0%
Full Refresh	57ms	110ms	56ms	1ms	0%

Results based on 10 iterations. P50 = median, P95 = 95th percentile.

---

Legend

• 🚀 Significant speedup (>100ms faster)
• ✅ Faster than baseline
• ➖ No significant change
• 🔺 Slower than baseline (>100ms)
• ⚠️ Significant slowdown (>500ms)

[github-actions]

Test Coverage Report (Linux)

Metric	Value
Current Coverage	79.5%
Base Branch Coverage	79.5%
Delta	0% ➖

---

Coverage unchanged.

[github-actions]

Performance Report (Windows) ➖

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta	Change
Server Startup	9ms	11ms	7ms	2ms	28.6%
Full Refresh	124ms	74261ms	92ms	32ms	34.8%

Results based on 10 iterations. P50 = median, P95 = 95th percentile.

---

Legend

• 🚀 Significant speedup (>100ms faster)
• ✅ Faster than baseline
• ➖ No significant change
• 🔺 Slower than baseline (>100ms)
• ⚠️ Significant slowdown (>500ms)

[github-actions]

Test Coverage Report (Windows)

Metric	Value
Current Coverage	76.51%
Base Branch Coverage	76.51%
Delta	0% ➖

---

Coverage unchanged.