update fails when /tmp is mounted noexec

I'd originally opened this under here https://github.com/astral-sh/uv/issues/17191 but apparently the issue is in axoupdater instead.

Step to reproduce...
$ `uv self update`
```
info: Checking for updates...
error: failed to execute installer
Caused by: Permission denied (os error 13)
```

Workaround is fairly straightforward...
$ `export TMPDIR="$HOME/.tmp" && mkdir -p "$TMPDIR" && uv self update`
```
info: Checking for updates...
success: Upgraded uv from v0.7.19 to v0.9.18! https://github.com/astral-sh/uv/releases/tag/0.9.18
```

Reproduce `uv self update` with `/tmp` mounted `noexec`, then inspect axoupdater's installer and temporary-file handling. Use the reported `TMPDIR` workaround as the comparison case; done means the update succeeds without requiring an executable `/tmp`.

Written by the indexing model from the issue text.