Build a minimized Nix with MinGW

At this point many features are stripped out, but this works:

- Can run libnix{util,store,expr} unit tests
- Can run some Nix commands

Co-Authored-By volth <volth@volth.com>
Co-Authored-By Brian McKenna <brian@brianmckenna.org>

src/libstore/builtins/unpack-channel.cc

@@ -1,30 +0,0 @@
-#include "builtins.hh"
-#include "tarfile.hh"
-
-namespace nix {
-
-void builtinUnpackChannel(
-    const BasicDerivation & drv,
-    const std::map<std::string, Path> & outputs)
-{
-    auto getAttr = [&](const std::string & name) {
-        auto i = drv.env.find(name);
-        if (i == drv.env.end()) throw Error("attribute '%s' missing", name);
-        return i->second;
-    };
-
-    auto out = outputs.at("out");
-    auto channelName = getAttr("channelName");
-    auto src = getAttr("src");
-
-    createDirs(out);
-
-    unpackTarfile(src, out);
-
-    auto entries = readDirectory(out);
-    if (entries.size() != 1)
-        throw Error("channel tarball '%s' contains more than one file", src);
-    renameFile((out + "/" + entries[0].name), (out + "/" + channelName));
-}
-
-}