guix: Fix guile-ssh detection

See the upstream patch commit message. guile-ssh 0.18 inadvertently
changed the make-session API in a way that broke Guix’s configure-time
version check. This means Guix was built without guile-ssh support since
the guile-ssh upgrade, but ‘guix offload’ was broken anyway due to the
dlopen issue fixed in the previous commit.

pkgs/by-name/gu/guix/package.nix

@@ -71,6 +71,11 @@ stdenv.mkDerivation rec {
       patch = "security/0102-daemon-Sanitize-successful-build-outputs-prior-to-ex.patch";
       hash = "sha256-mOnlYtpIuYL+kDvSNuXuoDLJP03AA9aI2ALhap+0NOM=";
     })
+    (fetchpatch {
+      name = "fix-guile-ssh-detection.patch";
+      url = "https://git.savannah.gnu.org/cgit/guix.git/patch/?id=b8a45bd0473ab2ba9b96b7ef429a557ece9bf06c";
+      hash = "sha256-oYkgM694qPK8kqgxatkr4fj/GL73ozTNQADNyDeU6WY=";
+    })
   ];
 
   postPatch = ''
@@ -147,7 +152,7 @@ stdenv.mkDerivation rec {
       wrapProgram $f \
         --prefix GUILE_LOAD_PATH : "$out/${guile.siteDir}:$GUILE_LOAD_PATH" \
         --prefix GUILE_LOAD_COMPILED_PATH : "$out/${guile.siteCcacheDir}:$GUILE_LOAD_COMPILED_PATH" \
-        --prefix GUILE_EXTENSIONS_PATH : "${guile-ssh}/lib/guile/3.0/extensions:$GUILE_EXTENSIONS_PATH"
+        --prefix GUILE_EXTENSIONS_PATH : "${guile-ssh}/lib/guile/3.0/extensions"
     done
   '';