store: Write string length as a 32-bit integer.

* guix/store.scm (write-string): Write L as a 32-bit integer.  Bug
  introduced in 82c38fe64c ("store:
  Micro-optimize `write-string'.")

guix/store.scm

@@ -162,7 +162,7 @@
          (l (bytevector-length s))
          (m (modulo l 8))
          (b (make-bytevector (+ 8 l (if (zero? m) 0 (- 8 m))))))
-    (bytevector-u64-native-set! b 0 l)
+    (bytevector-u32-set! b 0 l (endianness little))
     (bytevector-copy! s 0 b 8 l)
     (put-bytevector p b)))