fix(ci): disable immutable installs for dependency-scan

Yarn Berry auto-enables immutable installs in CI, but this repo gitignores
yarn.lock, so the install fails. Match the pattern used in react.yml.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: joker23

.github/workflows/dependency-scan.yml

@@ -36,6 +36,8 @@ jobs:
 
       - name: Install dependencies
         run: yarn install
+        env:
+          YARN_ENABLE_IMMUTABLE_INSTALLS: 'false'
 
       - name: Generate SBOM
         uses: launchdarkly/gh-actions/actions/dependency-scan/generate-sbom@0a54234f88a428df4163234dbb23ddb7fee8b8ec # main