Feature request: Allow --no-push configuration through environment variable

[73VW]

Hello everyone,

I won't use the bug issue template because this is not a bug report but a request for new functionality.

We run our container builds using shell scripts and would prefer to be able to set an environment variable for skipping push when building the images in merge/pull requests for example.
That would allow to keep build script as they are and only set a variable from the job configuration.

My idea is to do the same as for the KANIKO_REGISTRY_MIRROR variable.
The variable could be called KANIKO_NO_PUSH for example.

I am not a Go developper so I have no idea how to parse environment variable but I guess it might quite similar to what is done here.
If someone can confirm, I could open a PR to add what's missing. If it is more complicated, I won't be able to help.

Linking #144 because it was the original issue about building without pushing.

Thanks in advance.
Cheers