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>

src/bun.js/ModuleLoader.zig

@@ -92,7 +92,7 @@ pub fn resolveEmbeddedFile(vm: *VirtualMachine, path_buf: *bun.PathBuffer, linux
         },
         else => {},
     }
-    return bun.path.joinAbsStringBuf(bun.fs.FileSystem.instance.fs.tmpdirPath(), path_buf, &[_]string{tmpfilename}, .auto);
+    return bun.path.joinAbsStringBuf(bun.fs.FileSystem.RealFS.tmpdirPath(), path_buf, &[_]string{tmpfilename}, .auto);
 }
 
 pub export fn Bun__getDefaultLoader(global: *JSGlobalObject, str: *const bun.String) api.Loader {