ci: actions/cache v1 has been deprecated, use v4

.github/workflows/imgtool.yaml

@@ -51,7 +51,7 @@ jobs:
       with:
         fetch-depth: 0
     - name: Cache pip
-      uses: actions/cache@v1
+      uses: actions/cache@v4
       with:
         path: ~/.cache/pip
         key: ${{ runner.os }}-pip