Build warnings on lib/posix/options/env.c

[aescolar]

Describe the bug
Depending on compile optimizations settings, lib/posix/options/env.c builds with warnings with default compiler settings.

To Reproduce
Steps to reproduce the behavior:

1. mkdir build; cd build
2. cmake -GNinja -DBOARD=nrf5340dk/nrf5340/cpuapp ../samples/posix/env/ -DCONFIG_SPEED_OPTIMIZATIONS=y
3. ninja
4. See error

Expected behavior
No build warnings

Impact
Either annoyance for users, or broken CI when this is triggered in CI (it is triggered downstream with some samples).

Logs and console output

../../../../../../../zephyr/lib/posix/options/env.c: In function 'getenv_r':
../../../../../../../zephyr/lib/posix/options/env.c:109:17: error: 'strncpy' specified bound depends on the length of the source argument [-Werror=stringop-truncation]
  109 |                 strncpy(buf, val, vsize);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~
../../../../../../../zephyr/lib/posix/options/env.c:104:25: note: length computed here
  104 |                 vsize = strlen(val) + 1;
      |                         ^~~~~~~~~~~
../../../../../../../zephyr/lib/posix/options/env.c: In function 'setenv':
../../../../../../../zephyr/lib/posix/options/env.c:191:17: error: 'strncpy' output truncated before terminating nul copying as many bytes from a string as its length [-Werror=stringop-truncation]
  191 |                 strncpy(environ[ret], name, nsize);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../../../../../zephyr/lib/posix/options/env.c:128:51: note: length computed here
  128 |         const size_t nsize = (name == NULL) ? 0 : strlen(name);
      |                                                   ^~~~~~~~~~~~
cc1: all warnings being treated as errors

Environment (please complete the following information):

• OS: Linux
• Toolchain: Zephyr SDK 0.16.5-1
• Commit SHA or Version used: Main as of today a495fd9