build: fix skipping of flaky tests on Travis

`PARALLEL_ARGS` is overwritten in the Makefile if `JOBS` is set. Use
`CI_JS_SUITES` instead.

PR-URL: https://github.com/nodejs/node/pull/27002
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

.travis.yml

@@ -49,4 +49,4 @@ matrix:
         - ./configure
         - make -j2 V=
       script:
-        - PARALLEL_ARGS='--flaky-tests=skip' make -j1 test
+        - CI_JS_SUITES='--flaky-tests=skip default' make -j1 test