fix(cli): increment __DOOMSTEP in elisp instead

Some shells (like ksh on SDF) may complain about $((...)) arithmetic
expansion syntax. Rather than wrestle with old shells, I'll offload this
trivial operation to elisp instead.

Close: #6970

bin/doomscript

@@ -57,7 +57,7 @@ fi
 # Some state that Doom's CLI framework needs to know about the terminal. Read
 # the comments at the top of bin/doom for explanations.
 export __DOOMPID="${__DOOMPID:-$$}"
-export __DOOMSTEP="$((__DOOMSTEP+1))"
+export __DOOMSTEP="${__DOOMSTEP:-0}"
 export __DOOMGEOM="${__DOOMGEOM:-$(tput cols 2>/dev/null)x$(tput lines 2>/dev/null)}"
 export __DOOMGPIPE="${__DOOMGPIPE:-$__DOOMPIPE}"
 export __DOOMPIPE=