mkosi: Preserve environment when running integration tests with sudo

Otherwise we won't detect we're running in Github Actions and will
show the wrong command to fetch the journal.

.github/workflows/mkosi.yml

@@ -182,7 +182,7 @@ jobs:
       run: meson compile -C build mkosi
 
     - name: Run integration tests
-      run: sudo meson test -C build --no-rebuild --suite integration-tests --print-errorlogs --no-stdsplit --num-processes "$(($(nproc) - 1))"
+      run: sudo --preserve-env meson test -C build --no-rebuild --suite integration-tests --print-errorlogs --no-stdsplit --num-processes "$(($(nproc) - 1))"
 
     - name: Archive failed test journals
       uses: actions/upload-artifact@v4