[ci] Continue workflow when Coveralls fail

This commit updates the tests workflow to continue
when the coveralls action fails as proposed by one
of their incidents:

https://status.coveralls.io/incidents/v5mcbrsbhgt4

Signed-off-by: Jose Javier Merchante <jjmerchante@bitergia.com>

Author: jjmerchante

.github/workflows/tests.yml

@@ -48,6 +48,7 @@ jobs:
 
     - name: Coveralls
       uses: coverallsapp/github-action@643bc377ffa44ace6394b2b5d0d3950076de9f63 # v2.3.0
+      continue-on-error: true
       with:
         coverage-reporter-version: "v0.6.9"
         flag-name: run ${{ join(matrix.*, ' - ') }}