test: fix flaky conditions for ppc64 SEA tests

In test status files, `$system` will be the OS and not the arch (which
would be `$arch`).

Add `test-single-executable-application-disable-experimental-sea-warning`
to the list of tests marked flaky on Linux ppc64le.

test/sequential/sequential.status

@@ -47,8 +47,9 @@ test-watch-mode-inspect: SKIP
 # https://github.com/nodejs/node/issues/41286
 test-performance-eventloopdelay: PASS, FLAKY
 
-[$system==ppc || $system==ppc64]
+[$system==linux && $arch==ppc64]
 # https://github.com/nodejs/node/issues/50740
+test-single-executable-application-disable-experimental-sea-warning: PASS, FLAKY
 test-single-executable-application-empty: PASS, FLAKY
 test-single-executable-application-snapshot-and-code-cache: PASS, FLAKY
 test-single-executable-application-snapshot: PASS, FLAKY