Kaniko allows to access undefined variables when cache is enabled

[apollo13]

Note to maintainers: Attached a patch in #1915 -- not sure how to test that though, hints welcome.

Actual behavior
Kaniko allows to access ARGS/ENV variables that are not yet defined when --cache=true is used

Expected behavior
Undefined variables should behave as one would expect from a shell

To Reproduce

1. Use this dockerfile:

FROM alpine as build
ARG TEST=123
RUN echo $TESTNOTSET
ARG TESTNOTSET=ups

2. Run with & without --cache

Without cache:

INFO[0002] Running: [/bin/sh -c echo $TESTNOTSET]       

INFO[0002] Taking snapshot of full filesystem...        

with cache:

INFO[0002] Running: [/bin/sh -c echo $TESTNOTSET]       
ups
INFO[0002] Taking snapshot of full filesystem...        

Additional Information

• Dockerfile: see above
• Build Context: empty
• Kaniko Image (fully qualified with digest) gcr.io/kaniko-project/executor:bde904349e9b7797d14f12c7a2657bf79145d18e-debug

Triage Notes for the Maintainers

Description	Yes/No
Please check if this a new feature you are proposing
Please check if the build works in docker but not in kaniko
Please check if this error is seen when you use --cache flag
Please check if your dockerfile is a multistage dockerfile

Note, this is probably related to #1910 because optimize() accumulates all ENV/ARG variables before they should be used