fix BUCK_CLASSPATH limit on linux

Summary: When running buck test ... the RemoteExecution tests fails with "Argument list too long".

Reason: The concatanation of paths used as BUCK_CLASSPATH is crossing the limit emposed by linux for the env variable values. (https://github.com/torvalds/linux/blob/master/include/uapi/linux/limits.h#L8)

How to reproduce the problem on a devserver:

• execute jshell

/usr/local/java-runtime/impl/11/bin/jshell

• paste the script that spawn a process with an env value

int test(String env, int size) throws Exception {
    var p = new ProcessBuilder("ls");
    p.environment().put(env, "b".repeat(size));
    return p.start().waitFor();
}

test("BUCK_CLASSPATH", 131_056)
// ok: 0

test("BUCK_CLASSPATH", 131_057)
// fail: 7 - Argument list too long

Reviewed By: bobyangyf