Build patched libfuse3 to find more fusermount binaries

[probonopd]

Build static libfuse3 with a patch to find fusermount in the following way:

1. Absolute path contained in the FUSERMOUNT_PROG environment variable
2. fusermount3 on the $PATH
3. fusermount on the $PATH (known to work for our purposes)
4. fusermount4...99 on the $PATH(speculative attempt at future-proofing; but better than not trying at all)

Closes #10

[probonopd]

Maybe we should search for the binaries in question on FUSERMOUNT_DIR instead on the $PATH for security reasons?

The reason I used $PATH is that we can never know where (exotic) distributions will put binaries a few years down the road.

[TheAssassin]

I have not had time to review your patch. At a first glance, there is a lot to discuss here, especially regarding software security. We should never worsen libfuse's.

[probonopd]

Successfully tested on AlmaLinux 9.1 (Lime Lynx) which comes with /usr/bin/fusermount but not with fusermount3:

./appimagetool-x86_64.AppImage --appimage-extract
unzip artifacts.zip # Use runtime from this PR
VERSION=1 ./squashfs-root/AppRun --runtime-file ./runtime-x86_64 ./squashfs-root/
./appimagetool-1-x86_64.AppImage # WORKS