While testing a Borg update (patch attached), I noticed that the
FUSERMOUNT_DIR hack in Fuse 3 doesn't seem to work like it does with
Fuse 2.

The 1.2.0b3 beta of the upcoming Borg release can use either Fuse 2
(with python-llfuse) or Fuse 3 (with python-pyfuse3).

The `borg mount` command works as expected with Fuse 2 / llfuse.

But, with Fuse 3 / pyfuse3, it fails with:

fuse: failed to exec fusermount3: No such file or directory

When I commented out the FUSERMOUNT_DIR [0] substitution in the fuse-3
package and rebuilt Borg, `borg mount` instead gives us this, which is
expected, because this system does not have a setuid fusermount3:

fusermount3: mount failed: Operation not permitted

So, the substitution doesn't seem to help with Fuse 3: it just breaks
the lookup.

You can apply the patch and test it out. And you can also observe the
optimal behaviour if you switch the borg package's fuse implementation
from pyfuse3 to llfuse / Fuse 2.

Should we just remove the substitution in fuse-3?

[0]
https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/linux.scm?id=b09f2aa4dae7e2a2f0f37ba831a2428fee8557d2#n3306