core: Fix usroverlay regression

[cgwalters]

The /sysroot writability regressed this, and add a missing test case for this.

[gemini-code-assist]

Code Review

This pull request fixes a regression in bootc usroverlay where /sysroot was being unnecessarily made writable. The fix correctly detects the boot environment without invoking storage preparation logic that has side effects. A new test case is also added to cover the usroverlay functionality, verifying that it creates a transient writable overlay on /usr that is discarded on reboot. My review includes a couple of suggestions to improve the naming and description in the new test files for better clarity and maintainability.

[gemini-code-assist]

There's a typo in the summary. It should be usroverlay.

summary: Execute tests for bootc usroverlay

[jmarrero]

code lgtm, but it's weird that only stream10 is failing with:

  x Eval block failed with pipeline input
    ,-[/var/ARTIFACTS/work-readonly-testsjy4ax59d/tmt/plans/integration/readonly-tests/discover/default-0/tests/tmt/tests/booted/readonly/030-test-locking-read.nu:19:1]
 18 | # Await completion
 19 | 0..$n | each { |v|
    : ^^|^^
    :   `-- source value
 20 |     loop {
    `----

Error: nu::shell::non_zero_exit_code

  x External command had a non-zero exit code
    ,-[/var/ARTIFACTS/work-readonly-testsjy4ax59d/tmt/plans/integration/readonly-tests/discover/default-0/tests/tmt/tests/booted/readonly/030-test-locking-read.nu:26:9]
 25 |         # check status
 26 |         systemctl status $"bootc-status-($v)" out> /dev/null
    :         ^^^^|^^^^
    :             `-- exited with code 3
 27 |         # Clean it up
    `----

I am re-running these and see if it's some glitch as this nu test was not changed here.

[cgwalters]

OK right we need the same workaround here that landed in the factory reset test.