stdenvBootstrapTools: update aarch64 musl

The aarch64 musl bootstrap tools are woefully outdated.  Just getting
them to the point of being able to build new versions of themselves
required a number of hacks[1] that can be reverted once we have new
bootstrap tools, and before that it stdenv didn't even build for the
preceding three years.

[1]: NixOS#169764

So, following the script established by previous bootstrap tools
updates:

Files came from this Hydra build:

	https://hydra.nixos.org/build/246470544

…which used nixpkgs revision dd5621d
to instantiate:

	/nix/store/g480ass2vjmakaq03z7k2j95xnxh206a-stdenv-bootstrap-tools.drv

…and then built:

	/nix/store/95lm0y33dayag4542s8bi83s31bw68dr-stdenv-bootstrap-tools

I downloaded these files from Hydra and prefetched them into the Nix
store with the following commands:

	STOREPATH=95lm0y33dayag4542s8bi83s31bw68dr-stdenv-bootstrap-tools
	OPTIONS="--option binary-caches https://cache.nixos.org --option trusted-public-keys cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
	nix --extra-experimental-features nix-command store prefetch-file \
	  file://$(nix --extra-experimental-features nix-command store add-file --name bootstrap-tools.tar.xz  $(nix-store ${OPTIONS} -r /nix/store/${STOREPATH})/on-server/bootstrap-tools.tar.xz)
	nix --extra-experimental-features nix-command store prefetch-file --executable \
	  file://$(nix --extra-experimental-features nix-command store add-path --name busybox $(nix-store ${OPTIONS} -r /nix/store/${STOREPATH})/on-server/busybox)

These commands produced the following output:

	warning: you did not specify '--add-root'; the result might be removed by the garbage collector
	Downloaded 'file:///nix/store/fm8ys5bb737j82xgyiciyzpcwmzyi9b2-bootstrap-tools.tar.xz' to '/nix/store/jml0gh0q2rnc9sgr87irz0jfbl0fq545-fm8ys5bb737j82xgyiciyzpcwmzyi9b2-bootstrap-tools.tar.xz' (hash 'sha256-ZY9IMOmx1VOn6uoFDpdJbTnPX59TEkrVCzWNtjQ8/QE=').
	warning: you did not specify '--add-root'; the result might be removed by the garbage collector
	Downloaded 'file:///nix/store/rn9fx55mw2s0qg90zdjxa9xn11nlmjpg-busybox' to '/nix/store/9qylz8gqll63pprwkwlyfs9g4zilak2m-rn9fx55mw2s0qg90zdjxa9xn11nlmjpg-busybox' (hash 'sha256-WuOaun7U5enbOy8SuuCo6G1fbGwsO16jhy/oM8K0lAs=').

I used the hashes from the output above to create the fetchurl
invocation which is part of this commit.

I then started the bootstrap with the following command:

	nix --extra-experimental-features nix-command build -L -f . --arg localSystem '(import ./lib).systems.examples.aarch64-multiplatform-musl' hello

As @lovesegfault requested, here are the the sha256sums of all the
on-server components for extra verification:

	sha256sum /nix/store/${STOREPATH}/on-server/*

…which produced the following output:

	658f4830e9b1d553a7eaea050e97496d39cf5f9f53124ad50b358db6343cfd01  /nix/store/95lm0y33dayag4542s8bi83s31bw68dr-stdenv-bootstrap-tools/on-server/bootstrap-tools.tar.xz
	20cdfecb084ddb6b6b958f2b78fd2cc1d9641632f81ec7d5a48fae0a963ad0fa  /nix/store/95lm0y33dayag4542s8bi83s31bw68dr-stdenv-bootstrap-tools/on-server/busybox

pkgs/stdenv/linux/bootstrap-files/aarch64-unknown-linux-musl.nix

@@ -1,11 +1,25 @@
+#
+# Files came from this Hydra build:
+#
+#   https://hydra.nixos.org/build/246470544
+#
+# …which used nixpkgs revision dd5621df6dcb90122b50da5ec31c411a0de3e538
+# to instantiate:
+#
+#   /nix/store/g480ass2vjmakaq03z7k2j95xnxh206a-stdenv-bootstrap-tools.drv
+#
+# …and then built:
+#
+#   /nix/store/95lm0y33dayag4542s8bi83s31bw68dr-stdenv-bootstrap-tools
+#
 {
   busybox = import <nix/fetchurl.nix> {
-    url = "https://wdtz.org/files/wjzsj9cmdkc70f78yh072483x8656nci-stdenv-bootstrap-tools-aarch64-unknown-linux-musl/on-server/busybox";
-    sha256 = "01s6bwq84wyrjh3rdsgxni9gkzp7ss8rghg0cmp8zd87l79y8y4g";
+    url = "http://tarballs.nixos.org/stdenv/aarch64-unknown-linux-musl/dd5621df6dcb90122b50da5ec31c411a0de3e538/busybox";
+    sha256 = "sha256-WuOaun7U5enbOy8SuuCo6G1fbGwsO16jhy/oM8K0lAs=";
     executable = true;
   };
   bootstrapTools = import <nix/fetchurl.nix> {
-    url = "https://wdtz.org/files/wjzsj9cmdkc70f78yh072483x8656nci-stdenv-bootstrap-tools-aarch64-unknown-linux-musl/on-server/bootstrap-tools.tar.xz";
-    sha256 = "0pbqrw9z4ifkijpfpx15l2dzi00rq8c5zg9ghimz5qgr5dx7f7cl";
+    url = "http://tarballs.nixos.org/stdenv/aarch64-unknown-linux-musl/dd5621df6dcb90122b50da5ec31c411a0de3e538/bootstrap-tools.tar.xz";
+    hash = "sha256-ZY9IMOmx1VOn6uoFDpdJbTnPX59TEkrVCzWNtjQ8/QE=";
   };
 }