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>

arch/sandbox/cpu/os.c

@@ -35,6 +35,7 @@
 #include <asm/state.h>
 #include <os.h>
 #include <rtc_def.h>
+#include <env.h>
 
 /* Environment variable for time offset */
 #define ENV_TIME_OFFSET "UBOOT_SB_TIME_OFFSET"
@@ -1014,7 +1015,7 @@ long os_get_time_offset(void)
 {
 	const char *offset;
 
-	offset = getenv(ENV_TIME_OFFSET);
+	offset = env_get(ENV_TIME_OFFSET);
 	if (offset)
 		return strtol(offset, NULL, 0);
 	return 0;