Remove RAPIDS_DOCS_DIR from custom-job.yaml (#107)

In hindsight, this variable isn't needed at the workflow level since the
only workflow step that uses it is the last one.

Therefore we can just set the variable's value in the script that's run
in the last workflow step.

.github/workflows/custom-job.yaml

@@ -49,8 +49,6 @@ jobs:
     strategy:
       fail-fast: false
     runs-on: "linux-${{ inputs.arch }}-${{ inputs.node_type }}"
-    env:
-      RAPIDS_DOCS_DIR: /tmp/docs
     container:
       image: ${{ inputs.container_image }}
       env: