Hadrian: don't try to build "unix" on Windows
Since eb031a5b we try to build the unix
package with hadrian-ghc-in-ghci
, which meant that everything became unbuildable on Windows. See #21899 (closed).
Not sure what test should be added to ensure that this doesn't regress in the future. Seems perhaps overkill to add a Windows ghc-in-ghci test?