Use `fusermount3` everywhere

Problem:

fusermount3 (provided by the fuse3 debian package) or fusermount (fuse) is needed to build the images.

It's not installed by the bootstrap script.

Solution:

Add the fuse3 package to the bootstrap script.

We should try to install fuse3 on the developer's machine and not fuse because:

• Popular packages such as flatpak depend on fuse3.
• fuse3 is already installed on standard desktop or cloud installations.

Also move the podman and CI builds to fuse3 in order to minimize differences between build environments.

Changes introduced by this pull request:

• Add fuse3 to the bootstrap script.
• Change from fuse to fuse3 in podman and CI.

Related:

I tried adding fuse in !1386 (merged), but this was later reverted in 31d3c828 because it broke flatpak.