bot PR workflow should not overwrite existing project status

[surchs]

Our bot PR mover workflow https://github.com/neurobagel/planning/blob/3c1a6e2f6ddc6724dbf8558946f4bc00157ab4ff/.github/workflows/global_move_bot_pr_to_board.yml runs twice a day and then

• finds all open PRs with the _bot label and
• moves them into the Community column on the board

This is great unless

• the PR is already on the board
• we have manually given the PR a meaningful status other than Community and want to keep it

The simplest solution would be to check if the PR already has a Status value set, and if so, leave it alone.

[github-actions]

We want to keep our issues up to date and active. This issue hasn't seen any activity in the last 75 days.
We have applied the _flag:stale label to indicate that this issue should be reviewed again.
When you review, please reread the spec and then apply one of these three options:

• prioritize: apply the flag:schedule label to suggest moving this issue into the backlog now
• close: if the issue is no longer relevant, explain why (give others a chance to reply) and then close.
• archive: sometimes an issue has important information or ideas but we won't work on it soon. In this case
  apply the someday label to show that this won't be prioritized. The stalebot will ignore issues with this
  label in the future. Use sparingly!

[github-actions]

We want to keep our issues up to date and active. This issue hasn't seen any activity in the last 75 days.
We have applied the _flag:stale label to indicate that this issue should be reviewed again.
When you review, please reread the spec and then apply one of these three options:

• prioritize: apply the flag:schedule label to suggest moving this issue into the backlog now
• close: if the issue is no longer relevant, explain why (give others a chance to reply) and then close.
• archive: sometimes an issue has important information or ideas but we won't work on it soon. In this case
  apply the someday label to show that this won't be prioritized. The stalebot will ignore issues with this
  label in the future. Use sparingly!