browse: Write 'packages.json' atomically.

* browse/web-interface.scm (request-packages-json-handler): Use
'with-atomic-file-output'.

browse/web-interface.scm

@@ -68,13 +68,14 @@
                                     ("version"  ,(package-version package))
                                     ("synopsis" ,(package-synopsis package))
                                     ("homepage" ,(package-home-page package)))))))
-        (with-output-to-file packages-file
-          (lambda _
+        (with-atomic-file-output packages-file
+          (lambda (port)
             (scm->json (map package->json
                             (lset-difference
                              (lambda (a b)
                                (string= (package-name a) b))
-                             all-packages %package-blacklist)))))
+                             all-packages %package-blacklist))
+                       port)))
         (when cache-timeout-exists?
           (delete-file cache-timeout-file))))
     (request-file-handler "packages.json")))