Build/Test Tools: Install WordPress Importer plugin when installing the Docker-based local environment.

The WordPress Importer plugin is now downloaded to the `tests/phpunit/data/plugins` directory when running `npm run env:install`.

This ensures that the PHPUnit test suite will not fail when the plugin is missing.

This also introduces a new `WP_IMPORTER_REVISION` variable to the `.env` file, to control the SVN revision that is checked out.

Props johnbillion.
Fixes #49720.

git-svn-id: https://develop.svn.wordpress.org/trunk@50285 602fd350-edb4-49c9-b593-d223f7449a82

.github/workflows/phpunit-tests.yml

@@ -24,7 +24,6 @@ jobs:
   # Performs the following steps:
   # - Cancels all previous workflow runs for pull requests that have not completed.
   # - Checks out the repository.
-  # - Checks out the WordPress Importer plugin (needed for the Core PHPUnit tests).
   # - Logs debug information about the runner container.
   # - Installs NodeJS 14.
   # - Sets up caching for NPM.
@@ -47,9 +46,6 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v2
 
-      - name: Checkout the WordPress Importer plugin
-        run: svn checkout -r 2387243 https://plugins.svn.wordpress.org/wordpress-importer/trunk/ tests/phpunit/data/plugins/wordpress-importer
-
       - name: Log debug information
         run: |
           echo "$GITHUB_REF"