feat(npm): support a private registry token

.github/workflows/node-pr.yaml

@@ -36,11 +36,25 @@ on:
         required: false
         type: boolean
         default: false
+      npm-registry-scope:
+        description: "Scope served by a private npm registry, for example '@fortawesome'. Used only when npm-registry-url and the npm-registry-token secret are also set."
+        required: false
+        type: string
+        default: ""
+      npm-registry-url:
+        description: "URL of the private npm registry, for example 'https://npm.fontawesome.com/'. Used only when npm-registry-scope and the npm-registry-token secret are also set."
+        required: false
+        type: string
+        default: ""
       runner:
         description: "Runner label, or JSON object such as {\"group\":\"ci\",\"labels\":[\"linux\"]} to select a runner group. Falls back to vars.SHARED_ACTIONS_RUNNER, then ubuntu-latest."
         required: false
         type: string
         default: ""
+    secrets:
+      npm-registry-token:
+        description: "Auth token for the private npm registry"
+        required: false
 
 concurrency:
   group: node-pr-${{ github.repository }}-${{ github.event_name }}-${{ github.head_ref || github.ref_name }}-${{ inputs.working-directory }}
@@ -120,6 +134,21 @@ jobs:
         with:
           node-version: ${{ inputs.node-version }}
 
+      # The npmrc lives in RUNNER_TEMP, not the checkout, so no git command can pick the token up.
+      - name: Configure the private npm registry
+        env:
+          NPM_SCOPE: ${{ inputs.npm-registry-scope }}
+          NPM_URL: ${{ inputs.npm-registry-url }}
+          NPM_TOKEN: ${{ secrets.npm-registry-token }}
+        run: |
+          [ -n "$NPM_SCOPE" ] && [ -n "$NPM_URL" ] && [ -n "$NPM_TOKEN" ] || exit 0
+          echo "::add-mask::$NPM_TOKEN"
+          url="${NPM_URL%/}/"
+          npmrc="$RUNNER_TEMP/private-registry.npmrc"
+          (umask 077 && printf '%s:registry=%s\n//%s:_authToken=%s\n' "$NPM_SCOPE" "$url" "${url#*://}" "$NPM_TOKEN" > "$npmrc")
+          echo "NPM_CONFIG_USERCONFIG=$npmrc" >> "$GITHUB_ENV"
+          echo "Using a private npm registry for $NPM_SCOPE."
+
       - name: Install dependencies
         working-directory: ${{ inputs.working-directory }}
         env: