Nested environments to scale tikzpicture

Question

I found this nice solution to scale a TikZ picture to \textwidth and wanted to use it within an own defined environment.

\documentclass{article}
\usepackage{tikz}

\usepackage{environ}
\makeatletter
\newsavebox{\measure@tikzpicture}
\NewEnviron{scaletikzpicturetowidth}[1]{%
  \def\tikz@width{#1}%
  \def\tikzscale{1}\begin{lrbox}{\measure@tikzpicture}%
  \BODY
  \end{lrbox}%
  \pgfmathparse{#1/\wd\measure@tikzpicture}%
  \edef\tikzscale{\pgfmathresult}%
  \BODY
}
\makeatother

\newenvironment{bggrid}[1]{%
\begin{scaletikzpicturetowidth}{#1\textwidth}%
\begin{tikzpicture}[transform shape, scale=\tikzscale]
\draw (-2,-2) grid (2,2);
}
{\end{tikzpicture}\end{scaletikzpicturetowidth}}

\begin{document}

% works perfectly
% \begin{scaletikzpicturetowidth}{.5\textwidth}%
% \begin{tikzpicture}[transform shape, scale=\tikzscale]
% \draw (-2,-2) grid (2,2);
% \draw (0,0) circle (1);
% \end{tikzpicture}\end{scaletikzpicturetowidth}

% this causes the problem 
\begin{bggrid}{.5}
\draw (0,0) circle (1);
\end{bggrid}

\end{document}

But when I do so I obtain an error, I was not able to fix.

! LaTeX Error: \begin{scaletikzpicturetowidth} on input line 35 ended by \end{d
ocument}.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.39 \end{document}

? h
Your command was ignored.
Type  I <command> <return>  to replace it with another command,
or  <return>  to continue without it.

Does somebody has any idea how to solve this?