coveralls.yml: Do not run tests in parallel

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25105)

.github/workflows/coveralls.yml

@@ -72,7 +72,7 @@ jobs:
         cat /proc/cpuinfo
         ./util/opensslwrap.sh version -c
     - name: make test
-      run: make test HARNESS_JOBS=${HARNESS_JOBS:-4} TESTS='-test_external_krb5'
+      run: make test TESTS='-test_external_krb5'
     - name: generate coverage info
       run: lcov -d . -c
              --exclude "${PWD}/test/*"