cmake: avoid getcwd in CollapseFullPath

`CollapseFullPath` calls getcwd, which is a rather expensive system
call. we can replace it with `GetHomeOutputDirectory()` to save us from
the calling overhead

Source/cmAddCustomCommandCommand.cxx

@@ -215,7 +215,8 @@ bool cmAddCustomCommandCommand(std::vector<std::string> const& args,
       }
 
       if (cmSystemTools::FileIsFullPath(filename)) {
-        filename = cmSystemTools::CollapseFullPath(filename);
+        filename = cmSystemTools::CollapseFullPath(
+          filename, status.GetMakefile().GetHomeOutputDirectory());
       }
       switch (doing) {
         case doing_depfile: