Fetch hex.pm registry snapshot with external github repo.

This allows to avoid fetching registry file from S3 at build time,
making the build hermetic and much faster. Automatic tools will be used
to update said external repo content when Hex packages are
imported/update.

pkgs/development/erlang-modules/build-hex.nix

@@ -1,4 +1,4 @@
-{ stdenv, erlang, rebar3, openssl, libyaml, fetchurl }:
+{ stdenv, erlang, rebar3, openssl, libyaml, fetchurl, fetchFromGitHub }:
 
 { name, version, sha256
 , hexPkg ? name
@@ -14,7 +14,14 @@ stdenv.mkDerivation (attrs // {
 
   buildInputs = buildInputs ++ [ erlang rebar3 openssl libyaml ];
 
-  postPatch = ''
+  postPatch = let
+    registrySnapshot = fetchFromGitHub {
+      owner = "gleber";
+      repo = "hex-pm-registry-snapshots";
+      rev = "48147b0";
+      sha256 = "0ibfnhrhbka4n6wkhs99fpy3sjab54ip37jgvx2hcfhfr4pxhbxw";
+    };
+  in ''
     rm -f rebar rebar3
     if [ -e "src/${name}.app.src" ]; then
       sed -i -e 's/{ *vsn *,[^}]*}/{vsn, "${version}"}/' "src/${name}.app.src"
@@ -25,6 +32,9 @@ stdenv.mkDerivation (attrs // {
     echo "{plugins, [pc]}." >> rebar.config
     '' else ''''}
 
+    mkdir -p _build/default/{lib,plugins}/ ./.cache/rebar3/hex/default/
+    zcat ${registrySnapshot}/registry.ets.gz > .cache/rebar3/hex/default/registry
+
     ${postPatch}
   '';
 
@@ -44,7 +54,6 @@ stdenv.mkDerivation (attrs // {
     };
   in ''
     runHook preConfigure
-    mkdir -p _build/default/{lib,plugins}/
     ${concatMapStrings (dep: ''
       header "linking erlang dependency ${dep}"
       ln -s "${dep}" "_build/default/lib/${dep.packageName}"
@@ -62,7 +71,7 @@ stdenv.mkDerivation (attrs // {
   # this hermetic
   buildPhase = ''
     runHook preBuild
-    HOME=. rebar3 do update, compile
+    HOME=. rebar3 compile
     ${if compilePorts then ''
     HOME=. rebar3 pc compile
     '' else ''''}