Consider stackname from stack file

[KevyVo]

Right now we do not consider this case:

Let say the user has a stackfile name: beta in the .defang directory.

The contents of the beta file looks like this:

DEFANG_STACK = alpha
...

So once we process all the flags, env var and the env files. Our global object should have the stackfile final value as alpha. However, with the current logic, since the flag is set to beta. Should we stop at that value and treat it as the final result?