Add environment key support when building documentation

[Kyle-Ye]

Sometimes, due to various reasons (eg. upstream bug or complex business logic), we need to adjust some swiftsetting when building the documentation.

SwiftPackageIndex will set SPI_GENERATE_DOCS=1 when building the documentation for the package.

I hope we can do the similar thing for swiftinit.

[Kyle-Ye]

Any update on this?

A solution for me is add SWIFTINIT_BUILD=1 when buliding on swiftinit server.