job: avoid deriving env vars

[oliver-sanders]

There are two parts to the job script:

• The "task-specific" part.
• The "shared" part.

At the moment a lot of env vars are derived in the "shared" part using bash substitutions. These things could just be written out in the task-specific part.

I guess it wasn't done this way to make the task-specific part more minimal which makes sense for duplicate/deprecated vars e.g. CYLC_TASK_SUBMIT_NUMBER and CYLC_TASK_TRY_NUMBER, however, when values have to be derived by text wrangling it's not the nicest.

See #4201 (comment)