libsel4utils: remove unused environment setup

[anchao]

libsel4utils: remove unused environment setup

It is unnecessary to writing environment to the stack since the target environment size is always 0.

This PR also fix compile warning on native X86 build:

[2/8] Building C object apps/sel4test-driver/seL4_libs/libsel4utils/CMakeFiles/sel4utils.dir/src/process.c.obj
/home/archer/code/sel4/projects/seL4_libs/libsel4utils/src/process.c: In function ‘sel4utils_spawn_process_v’:
/home/archer/code/sel4/projects/seL4_libs/libsel4utils/src/process.c:320:13: warning: ‘sel4utils_stack_copy_args’ accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
  320 |     error = sel4utils_stack_copy_args(vspace, &process->vspace, vka, envc, envp, dest_envp, &initial_stack_pointer);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/archer/code/sel4/projects/seL4_libs/libsel4utils/src/process.c:320:13: note: referencing argument 5 of type ‘char *[0]’
/home/archer/code/sel4/projects/seL4_libs/libsel4utils/src/process.c:209:12: note: in a call to function ‘sel4utils_stack_copy_args’
  209 | static int sel4utils_stack_copy_args(vspace_t *current_vspace, vspace_t *target_vspace,
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~
[8/8] Generating ../../images/sel4test-driver-image-x86_64-pc99