Wie kann ich die Liniendicke der group-Klammern ändern?

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\documentclass[varwidth, margin=5mm]{standalone}
%\documentclass[a4paper]{article}
\usepackage[ngerman]{babel}
\usepackage{amsmath, relsize, tikz}
\usetikzlibrary{matrix}
\begin{document}
\begin{tikzpicture}
\matrix (m) [matrix of math nodes,
inner sep=0pt, column sep=0.25em, %row sep=0.1em,
nodes={inner sep=0.25em,text width=0.75em,align=center},
left delimiter={\lgroup}, right delimiter={\rgroup},
]
{
a_{1}  &  b_{1} &  c_{1} \\
a_{2}  &  b_{2} &  c_{2}\\
a_{3}  &  b_{3} &  c_{3}\\
};
\draw[]  (m-1-3.north west) -- (m-3-3.south west);
\end{tikzpicture}
\end{document}
 
 
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