Apply suggestions from code review

Co-authored-by: Norbert Elter <72046715+itsyoboieltr@users.noreply.github.com>

Author: Qbandev

README.md

@@ -42,7 +42,7 @@ This TypeScript module is maintained in the style of the MetaMask team.
   with:
     is-high-risk-environment: false
     yarn-custom-url: 'https://your-cdn.com/yarn-4.9.1/yarn.js#sha224.abc123'
-    yarn-install-max-retries: '7'
+    yarn-install-max-retries: 5
 ```
 
 ### Options

action.yml

@@ -28,7 +28,7 @@ inputs:
   yarn-install-max-retries:
     description: 'Maximum number of retries for the `yarn --immutable` install command.'
     required: false
-    default: '5'
+    default: 5
   yarn-custom-url:
     description: 'Optional custom URL for a Yarn bundle to be prepared and activated by Corepack. For CI testing.'
     required: false
@@ -87,10 +87,13 @@ runs:
     # Prepare and activate custom Yarn if URL is provided.
     # This runs *before* setup-node might try to cache a standard Yarn.
     - name: Prepare and activate custom Yarn
-      if: ${{ inputs.yarn-custom-url != '' }}
+      if: ${{ env.YARN_CUSTOM_URL }}
+      env:
+        COREPACK_ENABLE_UNSAFE_CUSTOM_URLS: 1
+        YARN_CUSTOM_URL: ${{ inputs.yarn-custom-url }}
       run: |
-        echo "Preparing and activating custom Yarn from URL: ${{ inputs.yarn-custom-url }}"
-        corepack prepare 'yarn@${{ inputs.yarn-custom-url }}' --activate
+        echo "Preparing and activating custom Yarn from URL: ${YARN_CUSTOM_URL}"
+        corepack prepare "yarn@${YARN_CUSTOM_URL}" --activate
       shell: bash
 
     # In a low-risk environment, try to download cache of `node_modules`, if it