Replace template’s `j1` with `deparallelize`

Solely because I think `ENV.deparallelize` is *way* clearer than
`ENV.j1`, at least to the uninitiated.

Also, updating the template will reduce the number of `ENV.j1`s that
persist because the Homebrew maintainers want clean histories of
formulae. Ideally, this change should prevent any more cases from being
introduced!

Closes #25431.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>