[Bug]: unmount command should be fusermount3 when building only against fuse3

[tinywrkb]

Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a bug that matches the one I want to file, without success.

flatpak-builder version

1.2.2

Flatpak version

1.12.7

How to reproduce

1. Build flatpak-builder only against fuse3, without fuse2.
2. No libfuse.2.so in the system, nor fusermount.
3. Starting building an application with flatpak-builder, and stop midway.

Expected Behavior

The temporary rofiles mount inside state-dir/rofiles/ should be unmounted.

Actual Behavior

flatpak-builder does not unmount the temporary rofiles mount on exit, and can't unmount it when starting a new build run.

Additional Information

Distro: Arch Linux.

Creating a fusermount symlink to fusermount3 fixes the issue, and it looks like that the unmount command in src/builder-context.c should be set as fusermount3 if only fuse3 support was enabled.

[nanonyme]

While this is not a bug, it seems like an important feature. Fedora has started hitting this with their fuse2 deprecation.

[smcv]

The equivalent of flatpak/flatpak@2cb17b4 is probably the best solution to this.