Make subprocess command logging atomic

### What is the problem or limitation you are having?

Each time a subprocess command is executed, `Subprocess` logs its command line, environment, return code, stdout/stderr, etc. However, these operations are independently serial; therefore, when subprocess calls are executed concurrently, the logging of different commands can become intermingled and undecipherable.

This is currently a problem for macOS code signing as all of the files are concurrently signed by a threadpool.

Example log file: https://github.com/user-attachments/files/16044976/briefcase.2024_06_30-10_13_00.package.log

### Describe the solution you'd like

Convert all of the subprocess command logging to be atomic such that each block of logging for a specific command is guaranteed to be on consecutive lines in the log.

### Describe alternatives you've considered

Omit concurrent subprocess execution.

### Additional context

_No response_

Start by locating the Subprocess implementation and its command, environment, return-code, stdout, and stderr logging. Reproduce concurrent subprocess execution, especially the macOS code-signing threadpool case, and verify that each command's complete log block remains on consecutive lines without interleaving.

Written by the indexing model from the issue text.