Fix disabling SHOPT_FIXEDARRAY (re: 2182ecfa)

It was easier than expected to fix this one. The many regression
test failures caused by disabling it were all due to one bug:
'typeset -p' output broke when building without this option.

src/cmd/ksh93/sh/nvtree.c: nv_attribute():
- In this function to print the attributes of a name-value pair,
  move four lines of code out of #if SHOPT_FIXEDARRAY...#endif that
  were inadvertently moved into the #if block in ksh93 2012-05-18.
  See the changes to nvtree.c in this multishell repo commit:
  aabab56a

src/cmd/ksh93/data/builtins.c:
- Update/rewrite 'typeset -a' documentation.
- Make it adapt to SHOPT_FIXEDARRAY.
- Fix a few typos.

src/cmd/ksh93/tests/arrays2.sh:
- Only one regression test needs a SHOPT_FIXEDARRAY check.

.github/workflows/ci.yml:
- Disable SHOPT_FIXEDARRAY when regression-testing without SHOPTs.
- Enable xtrace, add ':' commands for traced comments. This should
  make the CI runner output logs a little more readable.

src/cmd/ksh93/tests/arrays2.sh

@@ -222,7 +222,7 @@ compound cx
 typeset -a cx.ar[4][4]
 print -v cx > /dev/null
 print -v cx | read -C l 2> /dev/null || err_exit 'read -C fails from output of print -v'
-[[ ${cx%cx=} ==  "${l%l=}" ]] || err_exit 'print -v for compound variable with fixed 2d array not working'
+((SHOPT_FIXEDARRAY)) && [[ ${cx%cx=} != "${l%l=}" ]] && err_exit 'print -v for compound variable with fixed 2d array not working'
 
 # ======
 # Multidimensional arrays with an unset method shouldn't cause a crash.