Install referenced schema in "dependabot:validate" task

The "dependabot:validate" task 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
Dependabot configuration schema was recently updated to share resources with the "base" configuration schema, which
caused the validation to start failing:

schema /tmp/dependabot-schema-CuMFs6bqY1.json is invalid
error: can't resolve reference https://json.schemastore.org/base.json#/definitions/timezone from id https://json.schemastore.org/timezone

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

Taskfile.yml

@@ -364,6 +364,10 @@ tasks:
       SCHEMA_URL: https://json.schemastore.org/dependabot-2.0
       SCHEMA_PATH:
         sh: task utility:mktemp-file TEMPLATE="dependabot-schema-XXXXXXXXXX.json"
+      # Source: https://github.com/SchemaStore/schemastore/blob/master/src/schemas/json/base.json
+      BASE_SCHEMA_URL: https://json.schemastore.org/base.json
+      BASE_SCHEMA_PATH:
+        sh: task utility:mktemp-file TEMPLATE="base-schema-XXXXXXXXXX.json"
       # The Dependabot configuration file for the repository.
       DATA_PATH: ".github/dependabot.yml"
       # The asset Dependabot configuration files.
@@ -372,17 +376,20 @@ tasks:
         sh: task utility:mktemp-folder TEMPLATE="dependabot-validate-XXXXXXXXXX"
     cmds:
       - wget --quiet --output-document="{{.SCHEMA_PATH}}" {{.SCHEMA_URL}}
+      - wget --quiet --output-document="{{.BASE_SCHEMA_PATH}}" {{.BASE_SCHEMA_URL}}
       - |
         npx ajv-cli validate \
           --all-errors \
           --strict=false \
           -s "{{.SCHEMA_PATH}}" \
+          -r "{{.BASE_SCHEMA_PATH}}" \
           -d "{{.DATA_PATH}}"
       - |
         npx ajv-cli validate \
           --all-errors \
           --strict=false \
           -s "{{.SCHEMA_PATH}}" \
+          -r "{{.BASE_SCHEMA_PATH}}" \
           -d "{{.ASSETS_DATA_PATH}}"
 
   docs:generate: