Fix: Correct value cloning in os2._set_env for POSIX

The _set_env procedure in core/os/os2/env_posix.odin was
incorrectly cloning the 'key' argument for 'cval' instead of
the 'value' argument. This resulted in set_env effectively
setting the environment variable's value to its own key.

This commit corrects the typo to use the 'value' argument.

core/os/os2/env_posix.odin

@@ -30,7 +30,7 @@ _set_env :: proc(key, value: string) -> (err: Error) {
 	TEMP_ALLOCATOR_GUARD()
 
 	ckey := strings.clone_to_cstring(key, temp_allocator()) or_return
-	cval := strings.clone_to_cstring(key, temp_allocator()) or_return
+	cval := strings.clone_to_cstring(value, temp_allocator()) or_return
 
 	if posix.setenv(ckey, cval, true) != nil {
 		err = _get_platform_error_from_errno()