Use snprintf instead of sprintf in pg_regress.

To avoid static analyzers sounding the alarm, move to using snprintf
instead of sprintf. This was an oversight in 66d6086cbc.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/849588.1698179694@sss.pgh.pa.us

src/test/regress/pg_regress.c

@@ -845,7 +845,7 @@ initialize_environment(void)
 		{
 			char		s[16];
 
-			sprintf(s, "%d", port);
+			snprintf(s, sizeof(s), "%d", port);
 			setenv("PGPORT", s, 1);
 		}
 	}
@@ -867,7 +867,7 @@ initialize_environment(void)
 		{
 			char		s[16];
 
-			sprintf(s, "%d", port);
+			snprintf(s, sizeof(s), "%d", port);
 			setenv("PGPORT", s, 1);
 		}
 		if (user != NULL)