replace and remove run_command_v_opt_cd_env()

run_command_v_opt_cd_env() is only used in an example in a comment.  Use
the struct child_process member "env" and run_command() directly instead
and then remove the unused convenience function.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

tmp-objdir.h

@@ -10,9 +10,11 @@
  *
  * Example:
  *
+ *	struct child_process child = CHILD_PROCESS_INIT;
  *	struct tmp_objdir *t = tmp_objdir_create("incoming");
- *	if (!run_command_v_opt_cd_env(cmd, 0, NULL, tmp_objdir_env(t)) &&
- *	    !tmp_objdir_migrate(t))
+ *	strvec_push(&child.args, cmd);
+ *	strvec_pushv(&child.env, tmp_objdir_env(t));
+ *	if (!run_command(&child)) && !tmp_objdir_migrate(t))
  *		printf("success!\n");
  *	else
  *		die("failed...tmp_objdir will clean up for us");