Use codecov GitHub action:

Trying to resolve codecov not completing
successfully. The secrets.CODECOV_TOKEN was
added to the Tinkerbell org and is available
to all repos.

Signed-off-by: Jacob Weinstock <jakobweinstock@gmail.com>

.github/workflows/ci.yaml

@@ -43,7 +43,9 @@ jobs:
         run: nix-shell --run 'make ci'
 
       - name: upload codecov
-        run: bash <(curl -s https://codecov.io/bash)
+        uses: codecov/codecov-action@v4
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
 
       - name: compile binaries
         run: nix-shell --run 'make crosscompile'