BUGFIX: keep environment when plugin cache enabled

[ytsarev]

Description of your changes

Relates to #109 where we are modifying cmd.Env to enable terraformrc

Before this fix the environment would be stripped with default pluginCache: true

In standard k8s backend scenarion the Worskpace is failing with

Terraform encountered an error. Summary: Failed to initialize kubernetes configuration: invalid configuration: no configuration has been provided, try setting KUBERNETES_MASTER environment variable.

This change always preserves the environment for cmd.Env keeping filtering out of TF_PLUGIN_CACHE_DIR within the loop.

I have:

• [x] Run make reviewable to ensure this PR is ready for review.

How has this code been tested

 k apply -f examples/transition/00-mr-tf-workspace/workspace-inline.yaml

[ytsarev]

/test-examples="examples/transition/00-mr-tf-workspace/workspace-inline.yaml"

[ytsarev]

/test-examples="examples/workspace-inline-aws.yaml"