gnu: GnuTLS: Fix CVE-2020-11501.

* gnu/packages/tls.scm (gnutls)[replacement]: New field.
(gnutls-3.6.13): New variable.
(gnutls/guile-2.0, gnutls/dane, gnutls-3.6.10, gnutls3.0-gnutls): Use
PACKAGE/INHERIT.

gnu/packages/tls.scm

@@ -162,6 +162,7 @@ living in the same process.")
 (define-public gnutls
   (package
     (name "gnutls")
+    (replacement gnutls-3.6.13)
     (version "3.6.9")
     (source (origin
              (method url-fetch)
@@ -244,10 +245,23 @@ required structures.")
     (properties '((ftp-server . "ftp.gnutls.org")
                   (ftp-directory . "/gcrypt/gnutls")))))
 
-(define-public gnutls/guile-2.0
-  ;; GnuTLS for Guile 2.0.
+(define gnutls-3.6.13
   (package
     (inherit gnutls)
+    (version "3.6.A")
+    (source (origin
+              (method url-fetch)
+              (uri (string-append "mirror://gnupg/gnutls/v"
+                                  (version-major+minor version)
+                                  "/gnutls-3.6.13.tar.xz"))
+              (patches (search-patches "gnutls-skip-trust-store-test.patch"))
+              (sha256
+               (base32
+                "0f1gnm0756qms5cpx6yn6xb8d3imc2gkqmygf12n9x6r8zs1s11j"))))))
+
+(define-public gnutls/guile-2.0
+  ;; GnuTLS for Guile 2.0.
+  (package/inherit gnutls
     (name "guile2.0-gnutls")
     (inputs `(("guile" ,guile-2.0)
               ,@(alist-delete "guile" (package-inputs gnutls))))))
@@ -257,8 +271,7 @@ required structures.")
   ;; Authentication of Named Entities.  This is required for GNS functionality
   ;; by GNUnet and gnURL.  This is done in an extra package definition
   ;; to have the choice between GnuTLS with Dane and without Dane.
-  (package
-    (inherit gnutls)
+  (package/inherit gnutls
     (name "gnutls-dane")
     (inputs `(("unbound" ,unbound)
               ,@(package-inputs gnutls)))))
@@ -266,8 +279,7 @@ required structures.")
 (define gnutls-3.6.10
   ;; This is for 'guile3.0-gnutls', below.  Version 3.6.10 is the first to
   ;; introduce Guile 2.9/3.0 support.
-  (package
-    (inherit gnutls)
+  (package/inherit gnutls
     (version "3.6.10")
     (source (origin
               (inherit (package-source gnutls))
@@ -286,8 +298,7 @@ required structures.")
        ("util-linux" ,util-linux)))))
 
 (define-public guile3.0-gnutls
-  (package
-    (inherit gnutls-3.6.10)
+  (package/inherit gnutls-3.6.10
     (name "guile3.0-gnutls")
     (arguments
      (substitute-keyword-arguments (package-arguments gnutls-3.6.10)