[ci] Use C++20 also in the nightly build with cmake_cxxmodules=OFF

[guitargeek]

This avoids surprises where fragile test failures with C++17 and modules=OFF cannot be anticipated from the PR builds.

The test in question is in test_datatypes.py in the cppyy test suite:

=================================== FAILURES ===================================
____________________ TestDATATYPES.test49_addressof_method _____________________

self = <test.test_datatypes.TestDATATYPES object at 0x7fb5a77f3200>

    @mark.xfail(strict=True, condition=IS_MAC_ARM or IS_WINDOWS, reason="Crashes on mac-beta ARM64 and fails on Windows")
    def test49_addressof_method(self):
        """Use of addressof for (const) methods"""

        import cppyy

>       assert cppyy.addressof(cppyy.gbl.std.vector[int].at.__overload__(':any:', False))
E       AssertionError: assert 0
E        +  where 0 = <built-in function addressof>(<cppyy.CPPOverload object at 0x7fb5a7a62000>)
E        +    where <built-in function addressof> = <module 'cppyy' from '/github/home/ROOT-CI/build/lib/cppyy/__init__.py'>.addressof
E        +    and   <cppyy.CPPOverload object at 0x7fb5a7a62000> = <built-in method __overload__ of cppyy.CPPOverload object at 0x7fb5a7aaaa00>(':any:', False)
E        +      where <built-in method __overload__ of cppyy.CPPOverload object at 0x7fb5a7aaaa00> = <cppyy.CPPOverload object at 0x7fb5a7aaaa00>.__overload__
E        +        where <cppyy.CPPOverload object at 0x7fb5a7aaaa00> = <class cppyy.gbl.std.vector<int> at 0x5628463dfb40>.at

../../../../../../src/bindings/pyroot/cppyy/cppyy/test/test_datatypes.py:2348: AssertionError
----------------------------- Captured stderr call -----------------------------
input_line_412:2:106: error: 'RActivePhysicalColumns' is a protected member of 'ROOT::Internal::RPageSource'
 (vector<int>::reference (std::vector<int, std::allocator<int> >::*)(vector<ROOT::Internal::RPageSource::RActivePhysicalColumns::RColumnInfo>::size_type))&std::vector<int, std::allocator<int> >::at
                                                                                                         ^
/github/home/ROOT-CI/build/include/ROOT/RPageStorage.hxx:653:10: note: declared protected here
   class RActivePhysicalColumns {
         ^
=========================== short test summary info ============================

[guitargeek]

This implements a suggestion by @dpiparo made in #20980 (comment)

[github-actions]

Test Results

    22 files      22 suites   3d 13h 24m 17s ⏱️
 3 766 tests  3 766 ✅ 0 💤 0 ❌
75 816 runs  75 816 ✅ 0 💤 0 ❌

Results for commit efd3c2d.

♻️ This comment has been updated with latest results.

[pcanal]

This avoids surprises where fragile test failures with C++17 and modules=OFF cannot be anticipated from the PR builds.

Isn't that a platform we support though? (i.e. should both be in the PR builds). [Related: is the issue on our side or is it a compiler bug?]

[guitargeek]

To me, this looks like a compiler or cling issue. I mean it's pretty crazy that this test...

    def test49_addressof_method(self):
        """Use of addressof for (const) methods"""

        import cppyy

        assert cppyy.addressof(cppyy.gbl.std.vector[int].at.__overload__(':any:', False))
        assert cppyy.addressof(cppyy.gbl.std.vector[int].at.__overload__(':any:', True))

...fails with this error:

error: 'RActivePhysicalColumns' is a protected member of 'ROOT::Internal::RPageSource'

It's worth to open a GitHub issue about it I guess. I'll do that later.