LaTeX reader: Fix bug in macro resolution for environments.

This is a regression in pandoc 3.0 that affects environments
with arguments.  Closes #8573.

src/Text/Pandoc/Readers/LaTeX/Macro.hs

@@ -216,7 +216,7 @@ newenvironment = do
     let result = (name,
                     Macro GroupScope ExpandWhenUsed argspecs optarg
                       (bg:startcontents),
-                    Macro GroupScope ExpandWhenUsed argspecs optarg
+                    Macro GroupScope ExpandWhenUsed [] Nothing
                       (endcontents ++ [eg]))
     (do lookupMacro name
         case mtype of

test/command/8573.md

@@ -0,0 +1,7 @@
+```
+% pandoc -f latex -t plain
+\newenvironment{myenv}[1]{Open#1}{Close}
+\begin{myenv}{x}Hello\end{myenv}
+^D
+OpenxHelloClose
+```