🔥 (ci): Remove build and config caching

Instead, run make config each time to avoid issues with CMake and config
with different paths.

Also remove TIMESTAMP env variable that is not needed anymore.

.github/workflows/ci-build.yml

@@ -29,9 +29,7 @@ jobs:
       - name: Create cache variables
         id: create_cache_variables
         run: |
-          echo "TIMESTAMP=$(date +'%Y%m%d%H%M%S')" >> $GITHUB_ENV
           echo "::set-output name=timestamp::$(date +'%Y%m%d%H%M%S')"
-          echo "::set-output name=runner_name::$(basename $(dirname $(dirname $(dirname $(pwd)))))"
 
       - name: Cache ccache files
         id: cache_ccache_files
@@ -54,19 +52,9 @@ jobs:
         run: |
           make mbed_curl
 
-      - name: Cache cmake config & build directory
-        id: cache_cmake_config_and_build_directory
-        uses: actions/cache@v2
-        with:
-          path: |
-            _build/${{ matrix.custom_target }}
-            _build/cmake_config/${{ matrix.custom_target }}
-          key: ${{ runner.os }}-${{ steps.create_cache_variables.outputs.runner_name }}-cache-cmake_config_and_build_directory-${{ matrix.custom_target }}-${{ hashFiles('mbed_app.json', 'targets/custom_targets.json', '.mbed_version', 'Makefile', 'CMakeLists.txt') }}
-
       - name: Config build system
-        if: steps.cache_cmake_config_and_build_directory.outputs.cache-hit != 'true'
         run: |
-          make clean_config TARGET_BOARD=${{ matrix.custom_target }}
+          make config TARGET_BOARD=${{ matrix.custom_target }}
 
       - name: Ccache pre build
         run: |