fix: codecov does not allow '/' in the name of coverage flags (#1168)

Co-authored-by: Felix Angelov <felix@shorebird.dev>

.github/actions/dart_package/action.yaml

@@ -67,10 +67,18 @@ runs:
         dart pub global activate coverage
         dart test -j ${{inputs.concurrency}} --coverage=coverage --platform=${{inputs.platform}} && dart pub global run coverage:format_coverage --lcov --in=coverage --out=coverage/lcov.info --packages=.dart_tool/package_config.json --report-on=${{inputs.report_on}} --check-ignore
 
+    # Per https://stackoverflow.com/questions/73402042/github-action-expressions-split-string
+    - name: Split package name
+      env:
+        PACKAGE_PATH: ${{ inputs.working_directory}}
+      id: split
+      shell: ${{ inputs.shell }}
+      run: echo "package_name=${PACKAGE_PATH##*/}" >> $GITHUB_OUTPUT
+
     - name: Upload Coverage
       uses: codecov/codecov-action@v3
       with:
-        flags: ${{ inputs.working_directory }}
+        flags: ${{ steps.split.outputs.package_name }}
         token: ${{ inputs.codecov_token }}
 
     - uses: VeryGoodOpenSource/very_good_coverage@v2