fix(ci): set DEFAULT_BRANCH to current branch for super-linter

Super-linter defaults to diffing against the 'master' branch, but the
shallow checkout (fetch-depth: 1) does not fetch master. This caused a
FATAL error: 'Neither master, nor origin/master exist in /github/workspace'.

Setting DEFAULT_BRANCH to the current branch name avoids the need for
master to be present in the shallow clone.

Author: dgibbs64

.github/workflows/action-super-linter.yml

@@ -42,6 +42,7 @@ jobs:
         env:
           # To report GitHub Actions status checks
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          DEFAULT_BRANCH: ${{ github.ref_name }}
           VALIDATE_BIOME_FORMAT: false
           VALIDATE_BIOME_LINT: false
           VALIDATE_GITHUB_ACTIONS_ZIZMOR: false