Avoid selecting pre-releases for Python downloads without a version request (#7278)
Following #7263 the 3.13.0rc2 releases are at the top of the download
list but we should not select them unless 3.13 is actually requested.
Prior to this, `uv python install` would install `3.13.0rc2`.
```
❯ cargo run -- python install --no-config
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.14s
Running `target/debug/uv python install --no-config`
Searching for Python installations
Installed Python 3.12.6 in 1.33s
+ cpython-3.12.6-macos-aarch64-none
```
```
❯ cargo run -- python install --no-config 3.13
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.14s
Running `target/debug/uv python install --no-config 3.13`
Searching for Python versions matching: Python 3.13
Installed Python 3.13.0rc2 in 1.18s
+ cpython-3.13.0rc2-macos-aarch64-none
```