Also upgrade to nixpkgs 20.09

[hamishmack]

This should fix this error building ghc (that is currently breaking the eval of #2540):

rts/linker/PEi386.c:283:38: error:
     error: '__acrt_iob_func' undeclared here (not in a function); did you mean '__rts_iob_func'?
      283 | const void* __rts_iob_func = (void*)&__acrt_iob_func;
          |                                      ^~~~~~~~~~~~~~~
          |                                      __rts_iob_func
    |
283 | const void* __rts_iob_func = (void*)&__acrt_iob_func;
    |                                      ^
`x86_64-w64-mingw32-cc' failed in phase `C Compiler'. (Exit code: 1)
make[1]: *** [rts/ghc.mk:322: rts/dist/build/linker/PEi386.p_o] Error 1
make[1]: *** Waiting for unfinished jobs....
make: *** [Makefile:128: all] Error 2
builder for '/nix/store/b1j1lysc3ccwg7hl9y1shj09799zj7m4-x86_64-w64-mingw32-ghc-8.10.2.drv' failed with exit code 2

[mrBliss]

@hamishmack Thanks!