Bump ESP-IDF cache key

Builds are failing with current cache

Author: tannewt

.github/workflows/build.yml

@@ -489,7 +489,7 @@ jobs:
       id: idf-cache
       with:
         path: ${{ github.workspace }}/.idf_tools
-        key: ${{ runner.os }}-idf-tools-${{ hashFiles('.git/modules/ports/esp32s2/esp-idf/HEAD') }}-20210304
+        key: ${{ runner.os }}-idf-tools-${{ hashFiles('.git/modules/ports/esp32s2/esp-idf/HEAD') }}-20210415
     - name: Clone IDF submodules
       run: |
         (cd $IDF_PATH && git submodule update --init)