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GH-35029: [CI][C#] Install python on ubuntu-csharp image to fix nuget CI build (#35030)

### Rationale for this change

Fix the nuget CI nightly packaging failure.

### What changes are included in this PR?

Install python on ubuntu-csharp docker image

### Are these changes tested?

Tested on CI via archery

### Are there any user-facing changes?

No
* Closes: #35029

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>