texinfo6_8: init at 6.8 (without switching default)

(forward-picked from commit 1165533fc3, partially)
The point is to work around issue #139470: fetchurl on Hydra.

pkgs/development/tools/misc/texinfo/6.8.nix

@@ -0,0 +1,4 @@
+import ./common.nix {
+  version = "6.8";
+  sha256 = "1i7yb7mrp3inz25zbzv2pllr4y7d58v818f1as7iz8mw53nm7dwf";
+}

pkgs/development/tools/misc/texinfo/common.nix

@@ -54,6 +54,12 @@ stdenv.mkDerivation {
     && !stdenv.isDarwin
     && !stdenv.isSunOS; # flaky
 
+  checkFlagsArray = if version == "6.8" then [
+    # Test is known to fail on various locales on texinfo-6.8:
+    #   https://lists.gnu.org/r/bug-texinfo/2021-07/msg00012.html
+    "XFAIL_TESTS=test_scripts/layout_formatting_fr_icons.sh"
+  ] else null;
+
   meta = {
     homepage = "https://www.gnu.org/software/texinfo/";
     description = "The GNU documentation system";

pkgs/top-level/all-packages.nix

@@ -14878,6 +14878,7 @@ with pkgs;
   texinfo5 = callPackage ../development/tools/misc/texinfo/5.2.nix { };
   texinfo6_5 = callPackage ../development/tools/misc/texinfo/6.5.nix { }; # needed for allegro
   texinfo6 = callPackage ../development/tools/misc/texinfo/6.7.nix { };
+  texinfo6_8 = callPackage ../development/tools/misc/texinfo/6.8.nix { };
   texinfo = texinfo6;
   texinfoInteractive = appendToName "interactive" (
     texinfo.override { interactive = true; }