Print default installdir as part of warning.

Author: m-renaud

cabal-install/Distribution/Client/CmdInstall.hs

@@ -658,12 +658,13 @@ installExes verbosity baseCtx buildCtx platform compiler
 
       mkFinalExeName :: UnqualComponentName -> FilePath
       mkFinalExeName exe = prefix <> unUnqualComponentName exe <> suffix <.> exeExtension platform
-      installdirUnknown =
+      installdirUnknown path =
         "installdir is not defined. Set it in your cabal config file "
-        ++ "or use --installdir=<path>. Using default installdir."
+        ++ "or use --installdir=<path>. Using default installdir: " ++ show path
 
   installdir <- fromFlagOrDefault
-                (warn verbosity installdirUnknown >> pure installPath) $
+                (warn verbosity (installdirUnknown installPath)
+                  >> pure installPath) $
                 pure <$> cinstInstalldir clientInstallFlags
   createDirectoryIfMissingVerbose verbosity False installdir
   warnIfNoExes verbosity buildCtx