Add support for --project-dir

[patspam]

It would be great if this action had a dbt-project-dir option, for setting dbt's --project-dir flag.

My dbt project lives in a sub-directory of my repo, which means I can't use this action as-is.

I tried setting DBT_PROJECT_DIR as an environment variable which partially works, but unfortunately the python script looks for target/manifest.json in the root dir so this still fails.