ModifiableInstruction's getVariables() does not include variables in its flags

[rcjsuen]

Although it's not valid to use variables and expect it to be resolved, we should still include it in the list of returned variables from getVariables().

import { DockerfileParser } from 'dockerfile-ast';
let dockerfile = DockerfileParser.parse(
    "HEALTHCHECK --interval=$var CMD echo"
);
let instruction = dockerfile.getInstructions()[0];
console.log(instruction.getVariables().length);

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