1.86.0 build fails on NetBSD

```
libs/process/src/ext/cmd.cpp: In function 'boost::process::v2::shell boost::process::v2::ext::cmd(boost::process::v2::pid_type, boost::system::error_code&)':
libs/process/src/ext/cmd.cpp:315:61: error: could not convert 'vec' from 'std::vector >' to 'boost::process::v2::shell'
315 | if (!kd) {BOOST_PROCESS_V2_ASSIGN_LAST_ERROR(ec) return vec;}
| ^~~
| |
| std::vector >
libs/process/src/ext/cmd.cpp:326:12: error: could not convert 'vec' from 'std::vector >' to 'boost::process::v2::shell'
326 | return vec;
| ^~~
| |
| std::vector >

"g++" -fvisibility-inlines-hidden -pthread -O3 -finline-functions -Wno-inline -Wall -fvisibility=hidden -DBOOST_ALL_NO_LIB=1 -DBOOST_ASIO_NO_DEPRECATED -DBOOST_ATOMIC_STATIC_LINK=1 -DBOOST_COBALT_USE_STD_PMR=1 -DBOOST_FILESYSTEM_STATIC_LINK=1 -DBOOST_PROCESS_SOURCE=1 -DNDEBUG -I"." -c -o "bin.v2/libs/process/build/gcc-12/release/arm_64/link-static/pch-off/threading-multi/visibility-hidden/ext/cmd.o" "libs/process/src/ext/cmd.cpp"
```