[plugin] Use Swift SubProcess instead of Process when it will be available

[sebsto]

Expected behavior

The Plugins/AWSLambdaPackager/PluginUtils.swift struct should use SubProcess instead of Foundation's Process

https://github.com/apple/swift-foundation/blob/main/Proposals/0007-swift-subprocess.md

Actual behavior

The struct uses Process

Steps to reproduce

n/a

If possible, minimal yet complete reproducer code (or URL to code)

n/a

What version of this project (swift-aws-lambda-runtime) are you using?

main

Swift version

= 6.x

Amazon Linux 2 docker image version

No response