Fix `nix-channel --update` does not work on freshly-installed nix.

# Why it does not work by default

`nix-channel --update` does two main things:
1. fetch new tarball with nix expressions
2. unpack that tarball as part of the build system using
   a shell script derived by unpack-channel.nix.
3. install the result into /nix/store


[2.] fails because tarball unpacking runs in a sandboxed environment
a container with /nix/store and /bin/sh mounted.

unpack-channel.nix needs bash, tar and xz binaries in PATH.
These are normally provided by bash, tar and xz packages from /nix/store.

Unfortunately bash, tar and xz packages are not yet fetched as we
only have nix installed without nixpkgs tree.

# How we get it working

1. we patch unpack-channel.nix to only require 'sh'
2. we use /bin/sh as a static busybox and point nix at it:
   ./configure --with-sandbox-shell=/bin/busybox

Bug: https://bugs.gentoo.org/693482
Bug: https://github.com/NixOS/nix/issues/2673
--- a/corepkgs/unpack-channel.nix
+++ b/corepkgs/unpack-channel.nix
@@ -6,15 +6,9 @@ let
     ''
       mkdir $out
       cd $out
-      xzpat="\.xz\$"
-      gzpat="\.gz\$"
-      if [[ "$src" =~ $xzpat ]]; then
-        ${xz} -d < $src | ${tar} xf - ${tarFlags}
-      elif [[ "$src" =~ $gzpat ]]; then
-        ${gzip} -d < $src | ${tar} xf - ${tarFlags}
-      else
-        ${bzip2} -d < $src | ${tar} xf - ${tarFlags}
-      fi
+
+      tar xf $src
+
       if [ * != $channelName ]; then
         mv * $out/$channelName
       fi
@@ -26,7 +20,7 @@ in
 
 derivation {
   system = builtins.currentSystem;
-  builder = shell;
+  builder = "/bin/sh";
   args = [ "-e" builder ];
   inherit name channelName src;