Define PACKAGE_NAME env var in build-docs workflow

The 'Save built docs as an artifact' step references ${{ env.PACKAGE_NAME }}
in the artifact name, but the variable was never defined in this workflow,
producing an artifact named ' rendered documentation' with a leading space.

Define PACKAGE_NAME at the workflow level, matching the convention used in
conda-package.yml.

Author: antonwolfy

.github/workflows/build-docs.yml

@@ -7,6 +7,9 @@ on:
 
 permissions: read-all
 
+env:
+  PACKAGE_NAME: mkl_random
+
 jobs:
   build-and-deploy:
     name: Build and Deploy Documentation