Fix Flea tests in constrained builders

[ryanrhughes]

The RC builder has Bubblewrap installed, but its nested container cannot create an unprivileged namespace. Flea's tests treated "installed" as "usable", so eight sandbox integration tests failed before the package could build. Two thumbnailer fixture tests also assumed optional host thumbnailers that are not part of the build environment.

This keeps Flea's runtime sandbox unchanged. The package still fails closed when Bubblewrap cannot start.

check() now probes the same constrained Bubblewrap invocation Flea uses and skips only those eight integration tests when namespaces are unavailable. It also skips only the two host-thumbnailer fixtures when their optional programs are missing. The remaining Rust, QML, and keymap checks still run.

Validated in both environments:

• Normal local build: 336/336 Rust tests, 1,013 QML checks, and keymap checks pass.
• Isolated production RC builder: 326 Rust tests pass with the 10 environment-dependent tests filtered, 1,013 QML checks and keymap checks pass, and flea 0.1.2-2 packages successfully.

The pkgrel bump ensures the corrected build recipe produces a new immutable artifact.