ci: fix the nightly integration-tests job (missing deps + interpreter mix)

[bketelsen]

Summary

The Python Integration Tests jobs in the nightly workflow fail on every scheduled run (checked: every nightly since at least Jul 2 is red at ~1 minute). Two stacked problems, both in the integration job only:

1. Build never succeeded: the job's apt list was missing packages meson setup requires — gettext (msgfmt for i18n), desktop-file-utils (update-desktop-database), appstream — so Build GResources died first. The list now mirrors the UI-tests job's, which demonstrably builds the project on these runners.

2. pytest could never run: the job used actions/setup-python (putting a toolcache interpreter first in PATH) while installing its dependencies via apt into the system Python — No module named pytest regardless of the apt list. The workflow's other jobs are each internally consistent (unit tests: setup-python + pip install pytest; UI tests: system Python + apt packages); the integration job mixed the two models. It now follows the UI job's pattern, which the GTK/gi stack requires anyway.

Verification

Applied to a downstream fork (frostyard/bootc-installer) where the same workflow previously failed identically: the next workflow_dispatch run passed all 8 jobs (run). The integration tests themselves are root/FISHERMAN_BIN-gated e2e installs that skip in CI, so the job now builds, collects, and exits green.

🤖 Generated with Claude Code

[hanthor]

Approve. Both fixes are coherent: the added apt packages (gettext/msgfmt, desktop-file-utils, appstream) are genuinely required by meson setup for the GResources build, and dropping actions/setup-python so pytest resolves against the system Python where python3-pytest is installed fixes the interpreter/PATH mismatch. Aligning the integration job with the working UI-tests job is the right pattern for the GTK/gi stack. Fork-verified green. LGTM.