CI: join codecov array flags, instead of accessing it directly

closes: #16082

Signed-off-by: rhysd <lin90162@yahoo.co.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 2f285ff..f67f97c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -297,7 +297,7 @@
         if: matrix.coverage
         uses: codecov/codecov-action@v5
         with:
-          flags: linux,${{ matrix.features }}-${{ matrix.compiler }}-${{ matrix.extra }}
+          flags: linux,${{ matrix.features }}-${{ matrix.compiler }}-${{ join(matrix.extra, '-') }}
           token: ${{ secrets.CODECOV_TOKEN }}
 
       - name: ASan logs