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/unix/builtins/unpack-channel.cc

@@ -0,0 +1,30 @@
+#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));
+}
+
+}