Rename flows job env var.

[hjoliver]

This is a small change with no associated Issue.

Rename the job environment variable that holds flow numbers. The current name pre-dates integer flow labels, and is not consistent with other "number" variable names.

CYLC_TASK_FLOWS ---> CYLC_TASK_FLOW_NUMBERS

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.
• Already covered by existing tests.
• No change log entry required (why? e.g. invisible to users).
• (master branch) covered in documentation PR at Redo the User Guide Reflow section. cylc-doc#427