test: mark test-gc-http-client-timeout as flaky on arm

`sequential/test-gc-http-client-timeout` is failing on arm
frequently. Mark it as flaky to unblock PRs to land.

PR-URL: #43754
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

test/sequential/sequential.status

@@ -32,6 +32,8 @@ test-tls-psk-client: PASS, FLAKY
 test-tls-securepair-client: PASS, FLAKY
 
 [$arch==arm]
+# https://github.com/nodejs/node/issues/43638
+test-gc-http-client-timeout: PASS,FLAKY
 
 [$arch==s390x]
 # https://github.com/nodejs/node/issues/41286