app: swap ':' path separator to ';' on Windows.

The python.env file sets a ':' separator for GI_TYPELIB_PATH environment
variable, which breaks finding the correct typelib files in the CI, when also
manually setting a previous value for this environment variable.

I'm not entirely sure the rule of swapping ':' for ';' is universally true on
Windows, whichever the variable. Let's hope that it is.

app/plug-in/gimpenvirontable.c

@@ -341,6 +341,11 @@ gimp_environ_table_load_env_file (GimpEnvironTable *environ_table,
 
           separator = name;
           name = q + 1;
+
+#ifdef G_OS_WIN32
+          if (g_strcmp0 (separator, ":") == 0)
+            separator = ";";
+#endif
         }
 
       if (! gimp_environ_table_legal_name (name))