ci: add caching for imgtool pip packages

Cache python dependencies for faster install and test times.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/.github/workflows/imgtool.yaml b/.github/workflows/imgtool.yaml
index b5cb41b..c86d9e5 100644
--- a/.github/workflows/imgtool.yaml
+++ b/.github/workflows/imgtool.yaml
@@ -13,6 +13,11 @@
     - uses: actions/checkout@v2
       with:
         fetch-depth: 0
+    - name: Cache pip
+      uses: actions/cache@v1
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip
     - name: Install packages
       run: |
         export PATH="$HOME/.local/bin:$PATH"