Fetch pre-compiled rebar3 instead of building it locally

Advantages:

* faster build time (no build, actually)
* wider Erlang version range - compatibility
  (since it's build for Erlang <cur-2> by erlang/rebar3)

bin/install

@@ -25,30 +25,31 @@ install_rebar() {
 
   # Running this in a subshell because we don't to disturb the current
   # working directory.
-  (
-    cd "$(dirname "$source_path")" || exit 1
-    tar zxvf "$source_path" &>/dev/null || exit 1
-
-    cd "$(untar_path "$install_type" "$version" "$tmp_download_dir")" || exit 1
-
-    # Build rebar
-    ./bootstrap || exit 1
+  if version_3x_or_greater "$version"; then
+    (
+      cd "$(dirname "$source_path")" || exit 1
+
+      # Copy (executable) rebar3 into the bin directory inside the asdf install directory
+      chmod +x rebar3 || exit 1
+      mkdir -p "$install_path/bin/" || exit 1
+      cp rebar3 "$install_path/bin/" || exit 1
+    )
+  else
+    (
+      cd "$(dirname "$source_path")" || exit 1
+      tar zxvf "$source_path" &>/dev/null || exit 1
 
-    # Copy rebar into the bin directory inside the asdf install directory
-    mkdir -p "$install_path/bin/" || exit 1
+      cd "$(untar_path "$install_type" "$version" "$tmp_download_dir")" || exit 1
 
-    if version_3x_or_greater "$version"; then
-      local rebar3_binary="_build/default/bin/rebar3"
+      # Build rebar
+      ./bootstrap || exit 1
 
-      if [ ! -f $rebar3_binary ]; then
-        rebar3_binary="_build/prod/bin/rebar3"
-      fi
+      # Copy rebar into the bin directory inside the asdf install directory
+      mkdir -p "$install_path/bin/" || exit 1
 
-      cp "$rebar3_binary" "$install_path/bin/" || exit 1
-    else
       cp rebar "$install_path/bin/" || exit 1
-    fi
-  )
+    )
+  fi
 }
 
 download_source() {
@@ -72,6 +73,10 @@ get_download_file_path() {
 
   local pkg_name="${rebar_version}.tar.gz"
 
+  if version_3x_or_greater "$version"; then
+    pkg_name="rebar3"
+  fi
+
   echo "$tmp_download_dir/$pkg_name"
 }
 
@@ -100,7 +105,7 @@ get_download_url() {
   local version=$2
 
   if version_3x_or_greater "$version"; then
-    echo "https://github.com/erlang/rebar3/archive/${version}.tar.gz"
+    echo "https://github.com/erlang/rebar3/releases/download/${version}/rebar3"
   else
     echo "https://github.com/rebar/rebar/archive/${version}.tar.gz"
   fi