Correct check for existing Calculix

compile_all.py

@@ -1037,11 +1037,11 @@ def build_opencamlib(self, _: None):
     def build_calculix(self, _: None):
         """Cannot currently build Calculix (it's in Fortran, and we only support MSVC toolchain right now). Extract
         the relevant files from the downloaded zipfile and copy them"""
-        path_to_ccx_bin = os.path.join(os.getcwd(), "CL35-win64", "bin", "ccx", "218")
         if self.skip_existing:
-            if os.path.exists(path_to_ccx_bin):
+            if os.path.exists(os.path.join(self.install_dir, "bin", "ccx.exe")):
                 print("  Not rebuilding Calculix, it is already in the LibPack")
                 return
+        path_to_ccx_bin = os.path.join(os.getcwd(), "CL35-win64", "bin", "ccx", "218")
         if not os.path.exists(path_to_ccx_bin):
             raise RuntimeError("Could not locate Calculix")
         shutil.copytree(path_to_ccx_bin, os.path.join(self.install_dir, "bin"), dirs_exist_ok=True)