`ADDITIONAL_DOCC_ARGUMENTS: unbound variable` when running the Documentation check workflow

[bkhouri]

The .github/workflows/scripts/check-docs.sh script generated an unbound variable failure. Rightfully so, the variable is never defined. In addition, the repository does not have any documentation explaining how, or whether, the variable should be defined by the caller.

Steps to reproduce:

1. Clone this repository under the directory GitHub-workflows
2. Clone a Swift PM fork as a sibling to the GitHub workflow repository

    git clone -b t/6.4.x/gh10186_gha_update-doc-check-to-check-target https://github.com/bkhouri/swift-package-manager.git
   

3. Change directory to the SwiftPM repo and run the following:

   cd swift-package-manager
   ../github-workflows/.github/workflows/scripts/check-docs.sh
   

Actual

❯ ../github-workflows/.github/workflows/scripts/check-docs.sh
** swift-docc-plugin already exists
** Checking documentation targets...
** Checking target PackageManagerDocs...
../github-workflows/.github/workflows/scripts/check-docs.sh: line 59: ADDITIONAL_DOCC_ARGUMENTS: unbound variable