fix(decorations): Better unknown key error msg

Fully expand `\pgfkeyscurrentname` before being used in first-arg of
`/errors/unknown key`.

Co-authored-by: Henri Menke <henri@henrimenke.de>
Signed-off-by: muzimuzhi <muzimuzhi@gmail.com>

tex/generic/pgf/frontendlayer/tikz/libraries/tikzlibrarydecorations.code.tex

@@ -17,8 +17,10 @@
   /pgf/decoration/.unknown/.code=%
     \pgfifdecoration{\pgfkeyscurrentname}{\edef\tikz@decoration@name{\pgfkeyscurrentname}}
     {\pgfifmetadecoration{\pgfkeyscurrentname}{\edef\tikz@decoration@name{\pgfkeyscurrentname}}
+    % Fully expand `\pgfkeyscurrentname' before being used in first-arg of
+    % `/errors/unknown key'.
     {\pgfkeys{/errors/unknown
-        key={/pgf/decoration/\pgfkeyscurrentname}{#1}}}},%
+        key/.expanded={/pgf/decoration/\pgfkeyscurrentname}{\unexpanded{#1}}}}},%
   /pgf/decoration/raise/.code={\def\tikz@dec@shift{\pgftransformyshift{#1}}\tikz@dec@trans},
   /pgf/decoration/mirror/.code={%
     \csname if#1\endcsname