Install referenced schema in "Check npm" workflow

The "Check npm" GitHub Actions workflow validates the repository's `package.json` npm manifest file against its JSON
schema to catch any problems with its data format.

In order to avoid duplication of content, JSON schemas may reference other schemas via the `$ref` keyword. The
`package.json` schema was recently updated to share resources with the base schema, which caused the validation to start
failing:

schema /home/runner/work/_temp/json-schema/package-json-schema.json is invalid
error: can't resolve reference https://json.schemastore.org/base.json#/definitions/license from id https://json.schemastore.org/package.json#

The solution is to configure the workflow to download that schema as well and also to provide its path to the avj-cli
validator via a `-r` flag.

Author: per1234

.github/workflows/check-npm.yml

@@ -52,6 +52,16 @@ jobs:
           location: ${{ runner.temp }}/json-schema
           file-name: ava.json
 
+      # This schema is referenced by the package.json schema, so must also be accessible.
+      - name: Download base JSON schema
+        id: download-base-schema
+        uses: carlosperate/download-file-action@v2
+        with:
+          # See: https://github.com/SchemaStore/schemastore/blob/master/src/schemas/json/base.json
+          file-url: https://json.schemastore.org/base.json
+          location: ${{ runner.temp }}/json-schema
+          file-name: base.json
+
       # This schema is referenced by the package.json schema, so must also be accessible.
       - name: Download JSON schema for eslintrc.json
         id: download-eslintrc-schema
@@ -132,6 +142,7 @@ jobs:
           ajv validate \
             -s "${{ steps.download-schema.outputs.file-path }}" \
             -r "${{ steps.download-ava-schema.outputs.file-path }}" \
+            -r "${{ steps.download-base-schema.outputs.file-path }}" \
             -r "${{ steps.download-eslintrc-schema.outputs.file-path }}" \
             -r "${{ steps.download-jscpd-schema.outputs.file-path }}" \
             -r "${{ steps.download-npm-badges-schema.outputs.file-path }}" \