[SV 57896] Change directories before checking jobserver auth

[SV 57896] Change directories before checking jobserver auth

We want to process -C options as early as possible, before we might
write informational messages, so that Entering/Leaving messages have
the correct directory.

* src/main.c (main): Move code dealing with changing directories
before parsing of the jobserver auth flag.
* tests/scripts/features/jobserver: Test the order of enter/leave.