Potential dynamic build problem on Windows

[Xophmeister]

#859 has demonstrated that dynamic building in Windows can often fail in unusual ways. For example, this error doesn't make a huge amount of sense:

C:\Users\RUNNER~1\AppData\Local\Temp\.tmp2VQMTF\toml\src\parser.c : fatal error C1083: Cannot open compiler generated file: 'C:\Users\RUNNER~1\AppData\Local\Temp\.tmp2VQMTF\rust\parser.obj': Permission denied

This error -- which was a result of running topiary prefetch -- suggests that the TOML grammar couldn't be built because it was trying to link to the Rust grammar. The Rust grammar has been previously built and therefore its artefacts cleaned up. However, AFAIK, there's no dependency between the two grammars, so that's irrelevant.

Intuitively, this feels like a concurrency issue. Maybe the Windows linker keeps state, somehow, and cannot handle multiple builds concurrently 🤷

We should look into this. (Although it may be as easy as disabling the parallel feature on Windows builds.)

[Xophmeister]

#869 confirms that concurrent building is problematic in Windows. This issue is not solved, but we disable concurrent grammar builds on Windows to at least move forward.