chore: address gosec warnings (#1648)

## Checklist

- [x] I have read the [contribution
guidelines](https://github.com/ooni/probe-cli/blob/master/CONTRIBUTING.md)
- [x] reference issue for this pull request:
ooni/probe#2783
- [ ] if you changed anything related to how experiments work and you
need to reflect these changes in the ooni/spec repository, please link
to the related ooni/spec pull request: <!-- add URL here -->
- [ ] if you changed code inside an experiment, make sure you bump its
version number

<!-- Reminder: Location of the issue tracker:
https://github.com/ooni/probe -->

## Description

This diff addresses gosec warnings and tries to minimise them as much as
possible. The gosec warnings for the current `master` branch can be
found here:
https://github.com/ooni/probe-cli/actions/runs/10286957901/job/28468879578#step:6:1153.

internal/testingx/oonibackendwithlogin.go

@@ -234,7 +234,7 @@ func (h *OONIBackendWithLoginFlow) handleOpenVPNConfig() http.Handler {
 
 		// we must lock because of SetOpenVPNConfig
 		h.mu.Lock()
-		w.Write(h.openVPNConfig)
+		_, _ = w.Write(h.openVPNConfig)
 		h.mu.Unlock()
 	})
 }