SystemTools: remove 2 more explicit calls to FileIsDirectory()

SystemTools::FileExists() can already check this case. These instances were
forgotten in commit 0adafb51 (SystemTools: use
FindProgram() overload that checks for directory). The summary says
"FindProgram()", but it meant to be "FileExists()".

Change-Id: I5ef2919d915fffe3a70e3f47d0c49e18951e66eb