Rustc can't find windows tools in non-standard location

When building rust on windows for use in chromium, we get the following error:

error: linker `link.exe` not found
   |
   = note: program not found
 
 note: the msvc targets depend on the msvc linker but `link.exe` was not found
 
 note: please ensure that Visual Studio 2017 or later, or Build Tools for Visual Studio were installed with the Visual C++ option.
 
 note: VS Code is a different product, and is not sufficient.

The machine in question does have a windows toolchain installed, including link.exe, but it's not in the "normal" place. Rather than installing the SDK on our bots, we bundle all the windows tools together in a separate package, but as far as I can tell there's no way to inform the compiler of this fact.

Doing a little digging myself, it seems like the error is emitted here and the linker lookup happens here, which calls out to the find_msvc_tools crate, which then does a search based on the registry. Since our package isn't included in the registry, it doesn't get found.

It seems like the solution would be to pass a flag pointing to the location of the windows tools, e.g. something like clang's -winsysroot flag. I don't know how to go about adding that, however.

Start by reading compiler/rustc_codegen_ssa/src/back/link.rs at the reported error and linker.rs at the linker lookup, then inspect how find-msvc-tools searches for tools. Define how a flag for a non-standard Windows tool location should be passed through; done means a bundled toolchain containing link.exe can be found without registry installation.