env “TMPDIR" can't change the tmp store diretory

my codes

package main

import (
	"database/sql"
	"log"
	"os"

	_ "github.com/mattn/go-sqlite3"
)

func main() {
	os.Setenv("TMPDIR", "/mnt/test")
	db, err := sql.Open("sqlite3", "db.bak")
	if err != nil {
		log.Fatal(err)
	}
	defer db.Close()

	_, err = db.Exec("VACUUM")
	if err != nil {
		log.Fatal("Failed to vacuum database:", err)
	} else {
		log.Println("Database vacuumed successfully")
	}
}

❯ du -d1 -h db.bak
84M     db.bak

❯ df -h
/dev/loop0              1003K  931K  1.0K 100% /mnt/test

the result of it

azTempDirs[0] = NULL
azTempDirs[1] = /mnt/test
2024/02/21 20:40:06 Database vacuumed successfully

while sqlite3 report the error

❯ export TMPDIR=/mnt/test
❯ sqlite3 db.bak -cmd "VACUUM" .quit

Error: database or disk is full

which says sqlite3 cli used the specified TMPDIR but the code didn't.

Start by reproducing the provided Go program and sqlite3 CLI commands with TMPDIR set to /mnt/test, then compare their VACUUM temp-file behavior and the reported azTempDirs values. Trace the database/sql driver and SQLite VACUUM paths to identify the differing behavior; done when the discrepancy is corrected or clearly documented with a verified reproduction.