Migrate workflows from deprecated 'set-env' and 'add-path' to 'exportVariable' and 'addPath'

[matthewfeickert]

Description

GitHub Actions is deprecating the set-env and add-path commands in favor of environmental files with the new exportVariable and addPath commands.

Action and workflow authors who are setting environment variables via stdout should update any usage of the set-env and add-path workflow commands to use the new environment files.

The tag workflow currently relies on set-env and so will need to be migrated.

[matthewfeickert]

So, seems this can get replaced largely by

echo "{name}={value}" >> $GITHUB_ENV

[matthewfeickert]

(For reference I'm trying to debug the new workflow here https://github.com/matthewfeickert/throw-it-away-now)