create a simple yapapi example that passes an environment variable in a run command

[shadeofblue]

user stories:

• as a Golem creator, I'd like to know how I can pass environment variables to commands executed on the provider
• as an SDK team member, I'd like to confirm or deny the thesis: environment variables support works correctly

implementation details:

• use the env argument:
  https://github.com/golemfactory/yapapi/blob/master/yapapi/script/__init__.py#L154
• run e.g. /bin/sh -c set and check its output to verify that the command indeed sees the variable

outcomes:

• a new example in yapapi/examples folder which show users a way to set environment variables
• verify the relevant functionality works in yagna (vm runtime) -> as mentioned in create a simple yapapi example that passes an environment variable in a run command #1071 (comment) it possibly doesn't

[johny-b]

Just FYI, this just doesn't work for a while already:

#630

[approxit]

Example was created on branch https://github.com/golemfactory/yapapi/tree/1071-create-a-simple-yapapi-example-that-passes-an-environment-variable-in-a-run-command. Env variable is not picked up by yagna. See golemfactory/yagna#2448.