posix: env: Fix 2 build warnings

Fix 2 build warnings in posix/options/env:
The maximum length given to strncpy() matches the input
string length, which makes the call equivalent to strcpy().
As the destination buffer size has been ensured sufficient
(in the first case by chechking just before,
in the second case by allocating it big enough),
let's just use strcpy() instead.

lib/posix/options/env.c: In function 'getenv_r':
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);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~
lib/posix/options/env.c:104:25: note: length computed here
  104 |                 vsize = strlen(val) + 1;
      |                         ^~~~~~~~~~~
lib/posix/options/env.c: In function 'setenv':
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);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lib/posix/options/env.c:128:51: note: length computed here
  128 |         const size_t nsize = (name == NULL) ? 0 : strlen(name);
      |                                                   ^~~~~~~~~~~~

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>