Auto merge of #12635 - nc4rrillo:refactor-spawn-multiprocess, r=Manishearth

Extracted common parts of the starting content process

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I implemented a trait for both `process::Command` and `sandbox::Command` in order to constrain my generic `setup_common` method.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12622 (github issue number if applicable).

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- [X] These changes do not require tests because we are extracting common functionality across two code branches into a method

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