system: Avoid '_' as a pattern variable in 'match'.

* gnu/system.scm (operating-system-root-file-system): Don't use '_' as a
wildcard in 'match', to cope with literal semantics in 2.2.

gnu/system.scm

@@ -652,8 +652,8 @@ hardware-related operations as necessary when booting a Linux container."
 (define (operating-system-root-file-system os)
   "Return the root file system of OS."
   (find (match-lambda
-         (($ <file-system> _ _ "/") #t)
-         (_ #f))
+         (($ <file-system> device title "/") #t)
+         (x #f))
         (operating-system-file-systems os)))
 
 (define (operating-system-initrd-file os)