fix(cli): continue without network isolation when unavailable

[EmilienM]

Summary

When --network-isolation is given on a runner that cannot support it (for example Docker's default seccomp profile, which blocks the unshare syscall), fromager fails at startup with no workaround other than dropping the flag entirely.

This downgrades the hard failure to a warning and continues with isolation disabled for the rest of the run.

Testing

• tests/test_external_commands.py and tests/test_cli.py pass
• the changed path only executes on runners where network isolation is unavailable, so it is not reachable in CI

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Repository: python-wheel-build/fromager/.coderabbit.yaml
• Review profile: CHILL
• Plan: Advanced
• Run ID: 1c7baf7c-e746-4674-acd7-2bec5d07fbd5

📥 Commits

Reviewing files that changed from the base of the PR and between 45241aa and 8330d0a.

📒 Files selected for processing: 1

• src/fromager/__main__.py

Included review availability: This review used your included allowance. Your plan provides up to 1 included review per hour; 0 remain after this review.

📝 Walkthrough

When network isolation is requested but unsupported, main now logs a warning with the detection error, disables network isolation, and continues execution.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~8 minutes

Merge Risk: ⚪ Minimal · up to 8330d

The command warns and continues without network isolation when the runner cannot successfully execute the isolation probe. No concrete merge-blocking risk was established.

Pre-merge checks: 4

Check name	Status	Explanation
Description check		The description clearly explains the change: unsupported network isolation now produces a warning and the run continues without isolation. It also includes testing information.
Title check		The title clearly and concisely summarizes the primary change: the CLI continues when network isolation is unavailable.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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[EmilienM]

this was an experiment, do not merge it.