pg_regress: Disable autoruns for cmd.exe on Windows

This is similar to 9886744a36, to prevent the execution of other
programs due to autorun configurations which could influence the
postmaster startup.

This was originally applied on HEAD as of 83c75ac7fb69 without a
backpatch, but the patch has survived CI and buildfarm cycles.  I have
checked that cmd /d exists down to Windows XP, which should make this
change work correctly in the oldest branches still supported.

Discussion: https://postgr.es/m/20230922.161551.320043332510268554.horikyota.ntt@gmail.com
Backpatch-through: 12

src/test/regress/pg_regress.c

@@ -1126,7 +1126,7 @@ spawn_process(const char *cmdline)
 		comspec = "CMD";
 
 	memset(&pi, 0, sizeof(pi));
-	cmdline2 = psprintf("\"%s\" /c \"%s\"", comspec, cmdline);
+	cmdline2 = psprintf("\"%s\" /d /c \"%s\"", comspec, cmdline);
 
 	if ((restrictedToken =
 		 CreateRestrictedProcess(cmdline2, &pi)) == 0)