lint: Speed up the formatting linter.

By using go-to-location which caches the number of bytes to seek by to get to
specific lines in a file.

* guix/lint.scm (report-formatting-issues): Use go-to-location.

Change-Id: I34e4d3acfbb1e14e026d2e7f712ba8d22b56c147
Signed-off-by: Christopher Baines <mail@cbaines.net>

guix/lint.scm

@@ -1857,7 +1857,8 @@ them for PACKAGE."
 
   (call-with-input-file file
     (lambda (port)
-      (let loop ((line-number 1)
+      (go-to-location port starting-line 0)
+      (let loop ((line-number starting-line)
                  (last-line #f)
                  (warnings '()))
         (let ((line (read-line port)))