ci: enable coveralls parallel send

Since we use coveralls' [parallel webhook
API](https://docs.coveralls.io/api-parallel-build-webhook), it seems we should also set it
in our [plugin
config](https://github.com/emqx/coveralls-erl/blob/02daa4e5f157ff16d10b41ce00b829bc8734a9e1/src/rebar3_coveralls.erl#L132).

rebar.config.erl

@@ -565,6 +565,7 @@ coveralls() ->
                 {coveralls_service_job_id, os:getenv("GITHUB_RUN_ID")},
                 {coveralls_commit_sha, os:getenv("GITHUB_SHA")},
                 {coveralls_coverdata, "_build/test/cover/*.coverdata"},
+                {coveralls_parallel, true},
                 {coveralls_service_name, "github"}
             ],
             case