Switching from a `cargo-home` to `flat` layout breaks updates

In uv 0.4.30, we used the default install path resulting in the following receipt:

```json
{
"binaries": [
"uv",
"uvx"
],
"binary_aliases": {},
"cdylibs": [],
"cstaticlibs": [],
"install_layout": "cargo-home",
"install_prefix": "/Users/zb/.cargo",
"modify_path": true,
"provider": {
"source": "cargo-dist",
"version": "0.23.0"
},
"source": {
"app_name": "uv",
"name": "uv",
"owner": "astral-sh",
"release_type": "github"
},
"version": "0.4.30"
}
```

In uv 0.5.0, we switched to a `flat` layout by specifying target install paths. However, this results in unexpected update behavior. `uv self update` places the executables in `~/.cargo` instead of `~/.cargo/bin`. The previous version's executables are not removed. The receipt contains the new version so subsequent update attempts fail.

This appears to be caused by not loading the `install_layout` from the receipt at https://github.com/axodotdev/axoupdater/blob/d45201fcba097307272e1dc09be78471fe6efbc1/axoupdater/src/receipt.rs#L69-L71

The layout is not passed to the installer and it is hard-coded to `flat`: https://gist.github.com/zanieb/9dca39f62651087794bbb31fa377d72a#file-uv-installer-sh-L1018-L1020

The update results in the receipt:

```json
{
"binaries": [
"uv",
"uvx"
],
"binary_aliases": {},
"cdylibs": [],
"cstaticlibs": [],
"install_layout": "flat",
"install_prefix": "/Users/zb/.cargo",
"modify_path": true,
"provider": {
"source": "cargo-dist",
"version": "0.25.1"
},
"source": {
"app_name": "uv",
"name": "uv",
"owner": "astral-sh",
"release_type": "github"
},
"version": "0.5.0"
}
```

This may be related to https://github.com/axodotdev/axoupdater/issues/113

Start by tracing receipt loading in axoupdater/src/receipt.rs around lines 69-71 and the installer invocation shown around lines 1018-1020 of the linked script. Reproduce an update from the cargo-home layout to the flat layout, then verify that the selected layout is honored, old executables are removed, and the resulting receipt matches the installation.

Written by the indexing model from the issue text.