bp::environment::native_handle() cannot be compiled on Linux

Code example to reproduce the issue:

```c
#include

#include

namespace bp = boost::process;

int main() {
auto nativeEnv = boost::this_process::environment();
auto nativeEnvHandle = nativeEnv.native_handle();
std::cout << *nativeEnvHandle;

bp::environment env = nativeEnv;
// should also work, but don't
auto envHandle = env.native_handle();
std::cout << *envHandle;

return 0;
}
```

GodBolt link to reproduce the issue: https://godbolt.org/z/cT7ovGv68

Affected library versions: boost 1.77.0, boost 1.82.0 (another versions were not tested).
Affected compilers: gcc 6.3.0, gcc 13.2, clang 16.0.0 (another versions were not tested).

Compiler flags: `-std=c++11 -O2`

Compiler output (gcc 13.2):
```log
In file included from /opt/compiler-explorer/libs/boost_1_82_0/boost/process/environment.hpp:17,
from /opt/compiler-explorer/libs/boost_1_82_0/boost/process/env.hpp:10,
from /opt/compiler-explorer/libs/boost_1_82_0/boost/process.hpp:28,
from :1:
/opt/compiler-explorer/libs/boost_1_82_0/boost/process/detail/posix/environment.hpp: In instantiation of 'Char** boost::process::detail::posix::basic_environment_impl::native_handle() const [with Char = char; native_handle_type = char**]':
:14:39: required from here
/opt/compiler-explorer/libs/boost_1_82_0/boost/process/detail/posix/environment.hpp:213:54: error: cannot convert 'const __gnu_cxx::__alloc_traits >, std::__cxx11::basic_string >::value_type*' {aka 'const std::__cxx11::basic_string*'} to 'boost::process::detail::posix::basic_environment_impl::native_handle_type' {aka 'char**'} in return
213 | native_handle_type native_handle() const {return &_data.front();}
| ^~~~~~~~~~~~~~
| |
| const __gnu_cxx::__alloc_traits >, std::__cxx11::basic_string >::value_type* {aka const std::__cxx11::basic_string*}
Compiler returned: 1
```

Proposed solution: store the duplicate `vector _nativeData`. Every `char*` should point on the same element on `_data` vector. `_nativeData` should reload elements on every `_data` rebuilding or element (stored `std::string`) changing.