innoextract: pin to boost188 to fix the build

Without the chnage the build fails on `staging` as:

    Could not find a package configuration file provided by "boost_system"

pkgs/by-name/in/innoextract/package.nix

@@ -4,7 +4,7 @@
   fetchFromGitHub,
   cmake,
   makeWrapper,
-  boost,
+  boost188,
   xz,
   libiconv,
   withGog ? false,
@@ -25,7 +25,9 @@ stdenv.mkDerivation {
 
   buildInputs = [
     xz
-    boost
+    # pin to oplder boost188 as boost189
+    # fails to find cmake bits: https://github.com/dscharrer/innoextract/pull/199
+    boost188
   ]
   ++ lib.optionals stdenv.hostPlatform.isDarwin [ libiconv ];