perf(ci): cache npm dependencies in Cypress init job using buildjet cache

The Cypress init job ran npm ci from scratch on every invocation.
Add buildjet cache restore/save steps around npm ci, keyed on
package-lock.json hash, so subsequent runs with unchanged dependencies
skip the registry download entirely.

Uses buildjet/cache (v4.0.2) to match the existing context caching
already in this workflow.

Signed-off-by: Anna Larch <anna@nextcloud.com>
AI-Assisted-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala

.github/workflows/cypress.yml

@@ -73,11 +73,23 @@ jobs:
       - name: Set up npm ${{ steps.versions.outputs.npmVersion }}
         run: npm i -g 'npm@${{ steps.versions.outputs.npmVersion }}'
 
+      - name: Restore npm cache
+        uses: buildjet/cache/restore@3e70d19e31d6a8030aeddf6ed8dbe601f94d09f4 # v4.0.2
+        with:
+          path: ~/.npm
+          key: node-${{ steps.versions.outputs.nodeVersion }}-npm-${{ steps.versions.outputs.npmVersion }}-${{ hashFiles('**/package-lock.json') }}
+
       - name: Install node dependencies & build app
         run: |
           npm ci
           TESTING=true npm run build --if-present
 
+      - name: Save npm cache
+        uses: buildjet/cache/save@3e70d19e31d6a8030aeddf6ed8dbe601f94d09f4 # v4.0.2
+        with:
+          path: ~/.npm
+          key: node-${{ steps.versions.outputs.nodeVersion }}-npm-${{ steps.versions.outputs.npmVersion }}-${{ hashFiles('**/package-lock.json') }}
+
       - name: Save context
         uses: buildjet/cache/save@3e70d19e31d6a8030aeddf6ed8dbe601f94d09f4 # v4.0.2
         with: