Check for repository variable

[whikloj]

This PR starts a check for a Repository Variable called FCREPO_SPECIAL_RELEASE_TAG which has the format of fcrepo-X.Y.Z and matches a tag on the fcrepo/fcrepo repository.

You would set this variable on this repository under

Settings -> Security -> Secrets & Variables -> Actions

On the "Variables" tab, you would add a "Repository Variable" of FCREPO_SPECIAL_RELEASE_TAG and value of the release to be building.

For instance, right now it should be fcrepo-6.4.1.

Then when this runs it will use that variable instead of trying to find the most recent tagged version. Once we release a newly tagged version, the repository variable can be deleted and the action will revert to normal behaviour.

[tomcbe]

Thanks for tackling this.

Looks good to me.

[Surfrdan]

FCREPO_SPECIAL_RELEASE_TAG variable has been set to fcrepo-6.4.1 as requested