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>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>