cmSystemTools: Simplify call to FindProgram for our own executable

Remove the unnecessary `FindProgramPath` wrapper.  Call the KWSys
`FindProgram` directly, without our CMake-specific wrapper, since
we follow-up with a `ToNormalizedPathOnDisk` anyway.

Source/cmSystemTools.cxx

diff --git a/Source/cmSystemTools.cxx b/Source/cmSystemTools.cxx
index 248a8a0..a8691f3 100644
--- a/Source/cmSystemTools.cxx
+++ b/Source/cmSystemTools.cxx
@@ -3075,11 +3075,7 @@
     }
   }
 #else
-  std::string errorMsg;
-  std::string exe;
-  if (!cmSystemTools::FindProgramPath(argv0, exe, errorMsg)) {
-    // ???
-  }
+  std::string exe = cmsys::SystemTools::FindProgram(argv0);
 #endif
   exe = cmSystemTools::ToNormalizedPathOnDisk(std::move(exe));
   return exe;