pg_regress: Write processed input/*.source into output dir

Before, it was writing the processed files into the input directory,
which is incorrect in a vpath build.

src/test/regress/pg_regress.c

@@ -618,7 +618,7 @@ convert_sourcefiles_in(char *source_subdir, char *dest_dir, char *dest_subdir, c
 static void
 convert_sourcefiles(void)
 {
-	convert_sourcefiles_in("input", inputdir, "sql", "sql");
+	convert_sourcefiles_in("input", outputdir, "sql", "sql");
 	convert_sourcefiles_in("output", outputdir, "expected", "out");
 }