Use C++20 in CUDA 12 CI configuration

[msimberg]

Just testing.

[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: +0.07% (target: -1.00%)	:white_check_mark: ∅ (target: 90.00%)

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (d7662804beb15f33d834b83c49832fd37b5d08cc) | 18346 | 13763 | 75.02% | | | Head commit (41fa6dff029098cc0be733e08f3c7f7976adfe59) | 18346 (+0) | 13776 (+13) | 75.09% (**+0.07%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#1249) | 0 | 0 | **∅ (not applicable)** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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[msimberg]

At least CUDA 12.4.1 and GCC 13.3.0 seem to reproduce:

/pika/source/libs/pika/execution_base/include/pika/execution_base/any_sender.hpp(65): error: expression must have a constant value
      inline constexpr empty_vtable_t<T> empty_vtable{};
                                                     ^