Issue #1689: create unique temporary file with g_file_open_tmp().

Not sure this is really solving the issue reported, which is that
`g_get_tmp_dir()` uses environment variables (yet as g_file_open_tmp()
uses g_get_tmp_dir()…). But at least g_file_open_tmp() should create
unique temporary files, which prevents overriding existing files (which
is most likely the only real attack possible here, or at least the only
one I can think of unless some weird vulnerabilities exist in glib).