Add flatpak environment support

[jeffmaury]

In a flatpak environment, git cannot be executed directly but through flatpak-spawn

Flatpak environment is determined by environment variable FLATPAK_ID

[steveukx]

Is the intention here to prefix all spawned processes with flatpak-spawn git instead of just git?

[steveukx]

You may be able to use the new functionality for binary config to support flatpak:

https://github.com/steveukx/git-js/blob/main/docs/PLUGIN-CUSTOM-BINARY.md

[github-actions[bot]]

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