Fix version file creation for new repo name.

[hjoliver]

For compat with new repository name (cylc -> cylc-flow).

Following standard installation instructions, typing make in the unpacked release directory puts "flow-7.8.3" (e.g.) in the VERSION file, instead of just "7.8.3".

Which means, cylc --version will return flow-7.8.3!

No need for forward port: Cylc-8 version is determined differently.

This is a small change with no associated Issue.

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).
• (This change cannot really be tested - it affects unpacked releases only).
• Includes an appropriate entry in the release change log CHANGES.md.
• No documentation update required for this change.