Align temp directory resolution with os.tmpdir() (#25878)

## Summary
- Aligns Bun's temp directory resolution with Node.js's `os.tmpdir()`
behavior
- Checks `TMPDIR`, `TMP`, and `TEMP` environment variables in order
(matching Node.js)
- Uses `bun.once` for lazy initialization instead of mutable static
state
- Removes `setTempdir` function and simplifies the API to use
`RealFS.tmpdirPath()` directly

## Test plan
- [ ] Verify temp directory resolution matches Node.js behavior
- [ ] Test with various environment variable configurations
- [ ] Ensure existing tests pass with `bun bd test`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

scripts/runner.node.mjs

@@ -1128,6 +1128,7 @@ async function spawnBun(execPath, { args, cwd, timeout, env, stdout, stderr }) {
     ...process.env,
     PATH: path,
     TMPDIR: tmpdirPath,
+    BUN_TMPDIR: tmpdirPath,
     USER: username,
     HOME: homedir,
     SHELL: shellPath,