Install fuse3 alongside os-prober

Since version 1.77 os-prober probes unmounted partitions exclusively
through grub-mount and silently skips them when it cannot run. The grub
package ships grub-mount but only lists fuse3 as an optional
dependency, so without it os-prober is limited to already-mounted
partitions and misses the common dual-boot case of Windows living on
another disk. Installing fuse3 also lets grub-mount read NTFS in
userspace, keeping detection independent of kernel filesystem modules
inside the installation chroot.

archinstall/lib/installer.py

@@ -1432,7 +1432,9 @@ class Installer:
 			grub_default.write_text(config)
 
 		if os_prober:
-			self.pacman.strap('os-prober')
+			# fuse3 enables grub-mount, which os-prober requires to inspect
+			# partitions that are not mounted (e.g. Windows on another disk)
+			self.pacman.strap(['os-prober', 'fuse3'])
 
 			# grub-mkconfig only runs os-prober when GRUB_DISABLE_OS_PROBER is
 			# explicitly set to false; the stock config ships the option commented out