ardour_7: pin to boost186

Without the change the build fails as https://hydra.nixos.org/build/294958251:

    In file included from include/boost/math/tools/promotion.hpp:28,
                     from include/boost/math/special_functions/math_fwd.hpp:28,
                     from include/boost/math/special_functions/fpclassify.hpp:20:
    include/boost/math/tools/type_traits.hpp:208:12: error: ‘is_final’ has not been declared in ‘std’

ZHF: #403336

pkgs/applications/audio/ardour/7.nix

@@ -7,7 +7,7 @@
   fetchpatch2,
   alsa-lib,
   aubio,
-  boost,
+  boost186,
   cairomm,
   cppunit,
   curl,
@@ -127,7 +127,7 @@ stdenv.mkDerivation rec {
     [
       alsa-lib
       aubio
-      boost
+      boost186
       cairomm
       cppunit
       curl