Make release commit with deploy key

[milliams]

I created an SSH key locally with with:

ssh-keygen -q -t ed25519 -f ~/temp/doxylink_release_key -C '' -N ''

and in the doxylink GitHub settings page created a deploy key called "Release deploy key" with that key's public key and with write permissions. I put the private key part in an Actions secret called DEPLOY_KEY.

I then edited the branch protection ruleset to add Deploy Keys to the exception list.

This PR updates the release workflow to use that deploy key to check out the code so that when it pushes the tag and release commit it is allowed to bypass the PR requirement.

[JasperCraeghs]

Thank you so much! I'll create the releases by myself in the future unless you object.

[milliams]

I'll create the releases by myself in the future unless you object.

No problem at all. I'll make this release now to check that it's working, but I'm happy for you to make future releases whenever.