Run all workspace tests in CI (Windows)

[drinkcat]

Similar to #7392, it would be nice to run all tests in CI, on MacOS (basically adding workspace-tests: true to some windows jobs in the matrix).

When I tried in #7386, I hit this issue:

error: failed to run custom build command for `uu_stdbuf_libstdbuf v0.0.29 (D:\a\coreutils\coreutils\src\uu\stdbuf\src\libstdbuf)`
...
  src/libstdbuf.rs(18): error C2065: 'constructor': undeclared identifier
  src/libstdbuf.rs(18): error C2182: '__attribute': this use of 'void' is not valid
  src/libstdbuf.rs(19): error C2146: syntax error: missing ';' before identifier '__stdbuf_init'
  src/libstdbuf.rs(19): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
  src/libstdbuf.rs(21): warning C4508: '__stdbuf_init': function should return a value; 'void' return type assumed

I think libstdbuf.rs should simply not be built on Windows platform -- AFAICT nothing that is supported on Windows uses it.

Of course, there might be some more issues after this.