gnu: nss-certs: Do not inherit version and source from nss.

...in order to avoid a top-level variable reference.  This reverts commit
18c38c1898.

* gnu/packages/certs.scm (nss-certs)[version, source]: Do not inherit from NSS.

gnu/packages/certs.scm

@@ -71,8 +71,18 @@ (define certdata2pem
 (define-public nss-certs
   (package
     (name "nss-certs")
-    (version (package-version nss))
-    (source (package-source nss))
+    (version "3.67")
+    (source (origin
+              (method url-fetch)
+              (uri (let ((version-with-underscores
+                          (string-join (string-split version #\.) "_")))
+                     (string-append
+                      "https://ftp.mozilla.org/pub/mozilla.org/security/nss/"
+                      "releases/NSS_" version-with-underscores "_RTM/src/"
+                      "nss-" version ".tar.gz")))
+              (sha256
+               (base32
+                "0zyfi27lbdz1bmk9dmsivcya4phx25rzlxqcnjab69yd928rlm7n"))))
     (build-system gnu-build-system)
     (outputs '("out"))
     (native-inputs

gnu/packages/nss.scm

@@ -71,8 +71,8 @@ (define-public nspr
 in the Mozilla clients.")
     (license license:mpl2.0)))
 
-;;; Note: When updating, also test the build of the nss-certs package, which
-;;; uses the same source as nss.
+;;; Note: When updating, also update the nss-certs package, which cannot
+;;; inherit from here.
 (define-public nss
   (package
     (name "nss")