fix: "Unable to obtain node version." in git bash windows

$ nvm install-latest-npm
Attempting to upgrade to the latest working version of npm...
Unable to obtain node version.

nvm.sh

@@ -1273,7 +1273,7 @@ nvm_ls_current() {
     nvm_add_iojs_prefix "$(iojs --version 2>/dev/null)"
   elif nvm_tree_contains_path "${NVM_DIR}" "${NVM_LS_CURRENT_NODE_PATH}"; then
     local VERSION
-    VERSION="$(node --version 2>/dev/null)"
+    VERSION="$(command node --version 2>/dev/null)"
     if [ "${VERSION}" = "v0.6.21-pre" ]; then
       nvm_echo 'v0.6.21'
     else