Older versions of GCC fail to expand template parameter pack that appears in a lambda-expression

[lydia-at-amazon]

When trying to compile CETL with gcc 7.3.1, we ran into this compiler error:

/local/p4clients/pkgbuild-const/workspace/src/AdnExCyphalEmbeddedTemplateLibrary/include/cetl/pf17/variant.hpp:91:51: error: parameter packs not expanded with '...':
                         return std::forward<F>(fn)(std::integral_constant<std::size_t, Is>{},
                                ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                    std::forward<Args>(ar)...);
                                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~
/local/p4clients/pkgbuild-const/workspace/src/AdnExCyphalEmbeddedTemplateLibrary/include/cetl/pf17/variant.hpp:91:51: note:         'Is'
/local/p4clients/pkgbuild-const/workspace/src/AdnExCyphalEmbeddedTemplateLibrary/include/cetl/pf17/variant.hpp: In static member function 'static decltype(auto) cetl::pf17::detail::var::chronomorphize_impl<std::integer_sequence<long unsigned int, _Idx ...> >::lookup(F&&, std::size_t, Args&& ...)':
/local/p4clients/pkgbuild-const/workspace/src/AdnExCyphalEmbeddedTemplateLibrary/include/cetl/pf17/variant.hpp:93:22: error: expansion pattern '<lambda>' contains no argument packs
                     }...,
                      ^~~ 

Looks like it might be related to these GCC bug reports:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47226
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85305

Link to affected code: https://github.com/OpenCyphal/CETL/blob/main/include/cetl/pf17/variant.hpp#L83-L96