bug#54305: disk utility fail format fat

[Roman Riabenko <roman@riabenko.com>]

Hello

When initiating formatting of a USB flash drive in GNOME Files
(Nautilus), GNOME Disks (gnome-disk provided by gnome-disk-utility
package) opens followed by the volume formatting dialog. Currently, by
default, it is only possible to format the disk in Ext4. Other options
are grayed out, except for FAT, but choosing this option results in the
following error:

Error creating file system: Error spawning command-line `mkfs.vfat -l -
n "'/dev/sdb": Failed to execute child process "mkfs.vfat" (No such
file or directory) (g-exec-error-quark, 8) (udisks-error-quark, 0) 

(Here /dev/sdb is my flash drive.) I fixed this error by adding
dosfstools to the system profiles packages in config.scm:

(specification->package "dosfstools")

I see two issues here.

1. The FAT option was not grayed out in the formatting dialog. For
comparison, the NTFS option was grayed out until I added ntfs-3g to the
system profile too. May be GNOME Disks expects mkfs.vfat to be present,
so it does not check whether it is present like it does for other file
systems. 

So, it would be great for GNOME Disks to check whether mkfs.vfat is
available before proceeding like it does for other filesystems. It
could be easier to make dosfstools a dependency of gnome-disk-utility.
But, for comparison, the approach for UDisks seems to be to patch the
library instead of propagating system utilities:
https://issues.guix.gnu.org/41247#10

Possibly relevant patches:
https://issues.guix.gnu.org/40480
https://issues.guix.gnu.org/49128

2. GNOME Disks utility ignored the dosfstools package which I installed
in my user profile. For comparison, this applies to ntfs-3g too. In
relation to ntfs-3g with UDisks this seems to be expected behavior, but
it seemed to me as a bug at first:
https://guix.gnu.org/en/manual/devel/en/html_node/Desktop-Services.html#index-udisks_002dservice

I do not know what is necessary to make GNOME Disk utility recognize
the tools in the user profile and I am not sure it is necessary. It
just seemed against the spirit of guix that the user is forced to
reconfigure the system.

Roman