highlight: Ignore failure of chmod

The `chmod` makes sure the file is writable for the `open` call, but it
will fail if the file does not exist yet. After we ignore the failure,
the `open` will create the file correctly.
4 jobs for highlight-chmod in 6 minutes and 9 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #1214888
autotools-build

00:04:23

passed #1214889
meson-build

00:03:54

 
  Test
passed #1214890
autotools-test

00:01:46

33.0%
passed #1214891
meson-from-tarball

00:00:30