Add CODECOV_TOKEN now that tokeless uploads are no longer supported in v4

I verified that this specific secret is added to this repository.
diff --git a/.github/workflows/go.yml b/.github/workflows/go.yml
index a817e42..d659cf9 100644
--- a/.github/workflows/go.yml
+++ b/.github/workflows/go.yml
@@ -47,3 +47,6 @@
 
     - name: Upload coverage to codecov.io
       uses: codecov/codecov-action@6d798873df2b1b8e5846dba6fb86631229fbcb17 # v4.4.0
+      env:
+        CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+