sandbox: use env_get() for time offset instead of getenv()

sandbox: use env_get() for time offset instead of getenv()

The sandbox time offset is intended to be controlled via the U-Boot
environment, not the host process environment. Update os_get_time_offset()
to use env_get() instead of the libc getenv().

Leave other getenv() uses (e.g. U_BOOT_PERSISTENT_DATA_DIR,
UBOOT_SB_FUZZ_TEST) unchanged, since those refer to host environment
variables needed by sandbox tests.

Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>