feat(rustup-init/sh): allow setting RUSTUP_VERSION during installation

Author: rami3l

rustup-init.sh

@@ -87,7 +87,15 @@ main() {
             ;;
     esac
 
-    local _url="${RUSTUP_UPDATE_ROOT}/dist/${_arch}/rustup-init${_ext}"
+    local _url
+    if [ "${RUSTUP_VERSION+set}" = 'set' ]; then
+        say "\`RUSTUP_VERSION\` has been set to \`${RUSTUP_VERSION}\`"
+        _url="${RUSTUP_UPDATE_ROOT}/archive/${RUSTUP_VERSION}"
+    else
+        _url="${RUSTUP_UPDATE_ROOT}/dist"
+    fi
+    _url="${_url}/${_arch}/rustup-init${_ext}"
+
 
     local _dir
     if ! _dir="$(ensure mktemp -d)"; then