feat: added env_vars_file input to set env variables

[phbs1lva]

Added env_vars_file option to set environment variables.

Related to #234

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[verbanicm]

So two things on this...

1. gcloud exposes an --env-vars-file flag that could cause confusion to users

2. This deviates from how gcloud implements the --env-vars-file flag (details here).

I would much rather see the flags input used to set the --env-vars-file that is natively exposed by gcloud.