Panic during sysroot operations (git main)

[p5]

The current bootc version on git main (ae8930b) panics during bootc update, bootc status and any other bootc operation that interacts with the sysroot lock.

❯ rpm -qa | grep bootc  
bootc-202508212033.gae8930b1ad-1.fc42.x86_64

❯ bootc update                                                                             
⠁ Waiting for sysroot lock...                                                                                                                                                                                       
thread 'main' panicked at /builddir/build/BUILD/bootc-202508212033.gae8930b1ad-build/bootc-202508212033.gae8930b1ad/vendor/gio/src/gio_future.rs:66:13:
Spawning futures only allowed if the thread is owning the MainContext
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I do not yet know which commit introduced this as I've been pinned on an older image for about a week.

[p5]

This should also probably be followed up with an investigation as to why CI checks weren't able to catch these

[p5]

A clean cargo build on ae8930b does not produce an error.

I'm thinking the cause is likely rpm-ostree update running at the same time, although I thought this produced a different, more descriptive error. Will look at, at least, not making it panic and potentially improving the error

[jeckersb]

Hm I see this showing up in CI at the very least on aarch64 - https://artifacts.dev.testing-farm.io/1210de35-5213-4ba1-b8d1-7caa5b58d079/

thread 'main' panicked at /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/gio-0.20.12/src/gio_future.rs:66:13:
Spawning futures only allowed if the thread is owning the MainContext
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Error: nu::shell::core_dumped

  x External command core dumped
     ,-[/var/ARTIFACTS/work-test-20-local-upgradeogm5yztq/tmt/plans/integration/test-20-local-upgrade/discover/default-0/tests/tmt/tests/booted/test-image-pushpull-upgrade.nu:142:14]
 141 |     # Verify we staged an update
 142 |     let st = bootc status --json | from json
     :              ^^|^^
     :                `-- core dumped with SIGABRT (6)
 143 |     let staged_digest = $st.status.staged.image.imageDigest

[cgwalters]

Probably a regression from #1536 ...