Single-proc build breaks with npm_config_cache already defined

[asklar]

yarn sets up some environment variables (like npm_config_cache) and calls execSync. This calls the Win32 API CreateProcess and passes an environment block which is a merge of the existing environment block (i.e. from the calling process) with some additional stuff that includes npm_config_cache among others.

Environment variables in Windows are case-insensitive, but CreateProcess does not sanitize the environment block. This is a problem because if the calling process had set up NPM_CONFIG_CACHE (all caps), then yarn will just set the lowercase variant of the variable, and node will call CreateProcess which will contain the variable twice.
This breaks some tools that create dictionaries out of the environment block because they expect to run into each variable exactly once (regardless of casing).

run-windows is invoked via yarn, and it calls msbuild which crashes when trying to set up options for the compiler when it finds this option twice. We run into this when building in our CI in Azure DevOps pipeline, which sets up NPM_CONFIG_CACHE in the environment.

Related:

actions/runner-images#1566
dotnet/msbuild#5726
nodejs/node#35129
dotnet/runtime#42029
https://developercommunity.visualstudio.com/content/problem/1179475/multitooltask-should-sanitize-environment-variable.html
yarnpkg/yarn#8334