Remove a direct dependency on golang.org/x/sys

After #1477
has removed the last direct user of the package, update
also the dependency.

Don't completely remove it so that updating c/image can't
cause a downgrade of golang.org/x/sys in consumers;
eventually some dependency will start requiring a newer
version of golang.org/x/sys and the entry will completely
drop out (or we eventually update to the newer go.mod format
that lists indirect dependencies as well)

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

go.mod

@@ -39,6 +39,6 @@ require (
 	golang.org/x/crypto v0.0.0-20211215153901-e495a2d5b3d3
 	golang.org/x/net v0.0.0-20211112202133-69e39bad7dc2
 	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
-	golang.org/x/sys v0.0.0-20220128215802-99c3d69c2c27
+	golang.org/x/sys v0.0.0-20220128215802-99c3d69c2c27 // indirect
 	golang.org/x/term v0.0.0-20210927222741-03fcf44c2211
 )