Use C++20 in CUDA 12 CI configuration

[msimberg]

Just testing.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

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

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d766280)	18346	13763	75.02%
Head commit (41fa6df)	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: <coverage of head commit> - <coverage of common ancestor commit>

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: <covered lines added or modified>/<coverable lines added or modified> * 100%

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_{Codacy stopped sending the deprecated coverage status on June 5th, 2024. Learn more}

[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{};
                                                     ^