fix: respect $WATCHMAN_SOCK env variable for spawned service

[nekowinston]

Closes #1277.

I'm not really comfortable using C++, so apologies if the PR is overlooking some issues - this is merely the patch that worked for myself to fix the issue.

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@facebook-github-bot has imported this pull request. If you are a Meta employee, you can view this in D85015955. (Because this pull request was imported automatically, there will not be any future comments.)