bump esp-idf cache tag

.. we are seeing failures that we have previously diagnosed as a bad
cache.  The github ui provides no way to update or delete a cache, so
this is our workaround.

Author: jepler

.github/workflows/build.yml

@@ -467,7 +467,7 @@ jobs:
       id: idf-cache
       with:
         path: ${{ github.workspace }}/.idf_tools
-        key: ${{ runner.os }}-idf-tools-${{ hashFiles('.git/modules/ports/esp32s2/esp-idf/HEAD') }}-20210122
+        key: ${{ runner.os }}-idf-tools-${{ hashFiles('.git/modules/ports/esp32s2/esp-idf/HEAD') }}-20210128
     - name: Clone IDF submodules
       run: |
         (cd $IDF_PATH && git submodule update --init)