Don't discard environment variables in pipe logger

Before this patch `box.cfg { log = "|wrapper" }` started `wrapper` by
invoking /bin/sh -c 'wrapper' with empty environment. This patch makes
/bin/sh -c 'wrapper' invocation to inherit parent's environment.

Closes #3041

src/say.c

@@ -294,7 +294,6 @@ log_pipe_init(struct log *log, const char *init_str)
 	char cmd[] = { "/bin/sh" };
 	char args[] = { "-c" };
 	char *argv[] = { cmd, args, (char *) init_str, NULL };
-	char *envp[] = { NULL };
 	sigset_t mask;
 	sigemptyset(&mask);
 	sigaddset(&mask, SIGCHLD);
@@ -331,7 +330,7 @@ log_pipe_init(struct log *log, const char *init_str)
 		 * dies, we get SIGPIPE and terminate.
 		 */
 		setpgid(0, 0);
-		execve(argv[0], argv, envp); /* does not return */
+		execv(argv[0], argv); /* does not return */
 		diag_set(SystemError, "can't start logger: %s", init_str);
 		return -1;
 	}

test/app-tap/logger_pipe.result

+48

test/app-tap/logger_pipe.test.lua

+#!/usr/bin/env tarantool
+
+os.setenv('TEST_VAR', '48')
+box.cfg { log = '|echo $TEST_VAR' }
+os.exit(0)