CI only builds the reference manual on macos

[keyboardDrummer]

See the code

[davidcok]

The RM is written in Github markdown and published through GitHub pages to dafny.org.
Also, a pdf version is produced by using pandoc to convert markdown to Latex and thence to pdf.
The latter is done for each release in a GitHub action for the purpose. The Github action requires setting up an OS and environment and installing the panic and Latex tools. This works for versions of MacOS, but so far attempts to make a working environment on Ubuntu or Windows (even WSL) have failed. The issue is not particularly critical because all the project needs is a build of the pdf that works on some platform.

Slightly more important is that the Makefile (https://github.com/dafny-lang/dafny/blob/master/docs/DafnyRef/Makefile) that generates a between-releases version of the pdf only runs on MacOS. However, we are no longer publishing a bleeding-edge pdf of the RM, so this is only a problem for a developer who wants to check that the Makefile (which is the same as is used during release) produces a valid pdf (with correct formatting etc) but is not developing on MacOS.

At the moment this impediment is not a serious problem and so, while leaving this issue open, it is deemed low priority.