Update Pixi build customization example (#12499)

Hey! Was very happy to find a pixi example (thanks @pavelzw from #12155
:)) )

The example as it is though is broken saying "Missing configuration
option At least one of the following configuration options is required:
build.tools or build.commands." .

Adding 
```diff
+      tools:
+          python: "latest"
```
does placate the error message, but this doesn't really make sense since
Pixi manages its own Python.

Can we update the example linking to a relevant issue if there is one?

---------

Co-authored-by: Manuel Kaufmann <humitos@gmail.com>

Author: VeckoTheGecko

docs/user/build-customization.rst

@@ -525,6 +525,8 @@ Take a look at the following example:
 
    build:
       os: ubuntu-24.04
+      tools:
+          python: "latest"
       jobs:
          create_environment:
             - asdf plugin add pixi