fix build_systemd to use site_timeout - #994

What does this PR do?

build_systemd read max_timeout from the cluster config and passed it to
Launcher.new as max_timeout:, but the launcher's constructor accepts site_timeout:.
The unrecognized keyword was absorbed by **_, so @site_timeout was always nil.to_i,
i.e. 0.

script_timeout treats 0 as "no site limit configured", so it returned 'infinity' and
the site's timeout was never applied to any job.

build_systemd now reads site_timeout and falls back to max_timeout, passing it under
the name the launcher actually accepts.

max_timeout is still accepted because that is the name currently documented in
systemd.rst and the one sites have in their cluster configs today. A straight rename
would have silently stopped reading their existing configuration.

Related issue

Closes #912

Testing

• Tests included
• No tests needed — reason: ___

Four tests in systemd_test.rb: site_timeout reaches the launcher, max_timeout still
works, site_timeout wins when both are set, and an unset timeout is still 0.

The first three fail against master with Expected: 5678, Actual: 0, matching the debug
output in the issue.

Checklist

• Follows project code style and conventions
• Documentation provided (if new feature, adapter or behavior change) — see note below
• This is a large feature and was discussed in an issue first (if applicable) — N/A

Anything else?

Worth a release note. Sites that set max_timeout on a systemd cluster have had it
silently ignored. After this, their configured timeout starts being enforced — jobs that
previously ran without a site limit may begin hitting one. That is the intended behavior,
but it is a change those sites will notice.

Docs need a matching update in OSC/ood-documentation. systemd.rst currently mentions
timeout in prose and site_timeout in the example config, and neither matches the code
today. Not changed here since it's a separate repo.

linux_host has the same mismatch — build_linux_host passes max_timeout: to a launcher
that accepts site_timeout:. Filed separately rather in #995 than widening this PR.