libsecret: switch to gnutls

- libgcrypt being based on gnupg codebase has had a few "interesting" security incidents recently
- libsecret exposes `gnutls` as alternative crypto backend [1]
  - all tests still pass
  - only the implementation of `service_decode_aes_secret` is affected
    - this is not an exported symbol
  - this reduces closure size (gnutls is in the closure anyways, libgcrypt would be on-top after recent changes)
  - this reduces attack surface (gnutls is in the closure anyways, libgcrypt would be additional attack surface)
- after recent changes, libsecret is currently the largest consumer of libgcrypt

[1] a5cd57f103/meson.build (L40-58)

pkgs/by-name/li/libsecret/package.nix

@@ -12,7 +12,7 @@
   python3Packages,
   docbook-xsl-nons,
   docbook_xml_dtd_42,
-  libgcrypt,
+  gnutls,
   gobject-introspection,
   buildPackages,
   withIntrospection ?
@@ -106,7 +106,7 @@ stdenv.mkDerivation (finalAttrs: {
   ];
 
   buildInputs = [
-    libgcrypt
+    gnutls
   ]
   ++ lib.optionals withTpm2Tss [ tpm2-tss ]
   ++ lib.optionals abrmdSupport [ tpm2-abrmd ];
@@ -128,6 +128,7 @@ stdenv.mkDerivation (finalAttrs: {
     (lib.mesonBool "gtk_doc" withIntrospection)
     (lib.mesonBool "tpm2" withTpm2Tss)
     (lib.mesonOption "bashcompdir" "share/bash-completion/completions")
+    (lib.mesonOption "crypto" "gnutls")
   ];
 
   doCheck = stdenv.hostPlatform.isLinux && withIntrospection;