sh_setenviron(): deactivate compound assignment prefix

Reproducers:

$ ksh -c 'typeset -a arr=( ( (a $(($(echo 1) + 1)) c)1))'
ksh: echo: arr[0]._AST_FEATURES=CONFORMANCE - ast UNIVERSE - ucb: cannot be an array
ksh: [1]=1: invalid variable name
$ ksh -c 'typeset -a arr=( (a $(($(echo 1) + 1)) c)1)'
ksh: echo: arr._AST_FEATURES=CONFORMANCE - ast UNIVERSE - ucb: is not an identifier
ksh: [1]=1: invalid variable name

src/cmd/ksh93/sh/name.c: sh_setenviron():
- Save and clear the current compound assignment prefix (sh.prefix)
  while assigning to the _AST_FEATURES variable.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-06-22:
+
+- Fixed: 'echo' failed when used inside a command substitution that
+  is inside a nested compound assignment.
+
 2022-06-20:
 
 - Fixed a race condition that could cause redirections to fail with a