gitlab-ci: Pass --target explicitly to configure on Windows

Otherwise configure fails in the 32-bit case with
```
This GHC (c:/GitLabRunner/builds/8fc0e283/0/ghc/ghc/toolchain/bin/ghc) does not generate code for the build platform
   GHC target platform    : x86_64-unknown-mingw32
   Desired build platform : i386-unknown-mingw32
```