[Process] fix mustRun() in sigchild environments

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

When being run in sigchild environments, the sigchild compatibility mode needs to be enabled to be able to call `getExitCode()`. Since `mustRun()` uses `getExitCode()` to determine whether or not a process terminated successfully, it cannot be used in sigchild environments when the sigchild compatibility mode is disabled.