Add "fix" df command exiting with bogus status code

This commit addresses the issue documented in flatpak/xdg-desktop-portal#512
by using `df -x fuse.portal` as opposed to just `df`.

Fix sourced from:
https://www.mail-archive.com/desktop-packages%40lists.launchpad.net/msg651194.html

This appears to have come about because of an issue introduced
when upgrading to Linux Mint 20.3.

Signed-off-by: AKP <tom@tdpain.net>

internal/providers/disk.go

@@ -25,7 +25,7 @@ func NewDisk(mountPath string, okThreshold, warningThreshold float32) i3bar.Bloc
 }
 
 func (g *Disk) getAvailable() (float32, error) {
-	cmdout, err := runCommand("df")
+	cmdout, err := runCommand("df", "-x", "fuse.portal")
 	if err != nil {
 		return 0, err
 	}