Using GHA's `concurrency` syntax

[jakirkham]

Currently we have this logic to prevent multiple runs on main for different commits:

https://github.com/conda-forge/admin-requests/blob/cd3686599e10a654b7f4ec2a2cce46ab78db074b/.github/workflows/main.yml#L14-L20

Another option would be to use GHA's concurrency syntax. Here's an example.

If this seems reasonable, GHA configs in other repos may be able to employ the same strategy.