Structured bindings do not work with `environment::key_value_pair[_view]`

Structured bindings do not work with `environment::key_value_pair` and `environment::key_value_pair_view`. For example, the following code does not compile when using GCC 15.2.1 or Clang 21.1.6 on Linux:

```c++
#include
#include

int main() {
for (const auto [name, value] : boost::process::environment::current())
std::cout << name << '=' << value << '\n';
return 0;
}
```

Compiler output

```
$ g++ bug.cpp -lboost_process
bug.cpp: In function ‘int main()’:
bug.cpp:5:21: error: cannot decompose inaccessible member ‘boost::process::v2::environment::key_value_pair_view::value_’ of ‘const boost::process::v2::environment::key_value_pair_view’
5 | for (const auto [name, value] : boost::process::environment::current())
| ^~~~~~~~~~~~~
In file included from /usr/include/boost/process/environment.hpp:1,
from bug.cpp:2:
/usr/include/boost/process/v2/environment.hpp:435:20: note: declared private here
435 | string_view_type value_;
| ^~~~~~
$ clang bug.cpp -lboost_process
bug.cpp:5:21: error: type 'const key_value_pair_view' decomposes into 1 element, but 2 names were
provided
5 | for (const auto [name, value] : boost::process::environment::current())
| ^
/usr/include/boost/process/v2/environment.hpp:1377:14: note: selected 'begin' function with
iterator type 'iterator'
1377 | iterator begin() const {return iterator(handle_.get());}
| ^
1 error generated.
```

Interestingly, if `const auto` is replaced with just `auto`, GCC compiles the code without any warnings, while Clang still does not compile it but produces a different error message, that seems to explain the actual cause:

```
bug.cpp:5:15: error: 'value' is a private member of 'std::integral_constant'
5 | for (auto [name, value] : boost::process::environment::current())
| ^
/usr/include/boost/process/v2/environment.hpp:1280:82: note: constrained by implicitly private
inheritance here
1280 | class tuple_size : integral_constant {};
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib64/gcc/x86_64-suse-linux/15/../../../../include/c++/15/type_traits:94:28: note:
member is declared here
94 | static constexpr _Tp value = __v;
| ^
1 error generated.
```