github: Try to make the tests pass (re: 9ad9a1de, 5c389035)

So, the pty regression tests on the Linux GitHub runner all failed.

Let's test an assumption: the reason is that we need the stty
builtin to properly set the pty state, because the OS-provided stty
command does not work if there is no real tty.

src/cmd/ksh93/data/builtins.c:
- Compile in the stty built-in. This adds about 20k to the binary
  for a command that most users rarely need and even more rarely
  need to be built in, so only compile it in on non-release builds.

src/cmd/ksh93/tests/pty.sh:
- Skip the tests if we cannot either use the stty builtin or change
  the state of the real terminal to be compatible with the tests.

src/cmd/ksh93/data/builtins.c

@@ -153,6 +153,10 @@ const struct shtable3 shtab_builtins[] =
 #if !_std_malloc && !_AST_std_malloc
 	CMDLIST(vmstate)
 #endif
+#if !_AST_ksh_release
+	/* The pty.sh regression tests need the stty builtin */
+	CMDLIST(stty)
+#endif
 #endif
 #if SHOPT_REGRESS
 	"__regress__",		NV_BLTIN|BLT_ENV,	bltin(__regress__),