Small fix to `emitModuleJob` for compatibility with <=5.3 compilers.
Small fix to `emitModuleJob` for compatibility with <=5.3 compilers.

SwiftCI is using a 5.3 compiler and errors out on this code with:
```
swift-driver/Sources/SwiftDriver/Jobs/Planning.swift:188:31: error: variable used within its own initial value
               let emitModuleJob = try emitModuleJob()
```

apple/swift-driver,apple/swift-driver,apple/swift-driver,apple/swift-driver