feat: passthrough additional env vars to provider bin

[bavarianbidi]

as some provider binaries probably need additional environment variables set (e.g kubernetes as client-go depends on KUBERNETES_SERVICE_ vars) it should be possible to define a list of environment variables which should get bypassed into the provider binary execution

tested with our kubernetes-provider binary where i want to bypass the KUBERNETES_ variables into the binary execution call

[[provider]]
name = "kubernetes_external"
description = "kubernetes provider"
provider_type = "external"
[provider.external]
config_file = "/home/mb/bin/garm/garm-provider-k8s-config.yaml"
provider_executable = "/home/mb/bin/garm/garm-provider-k8s"
environment_variables = [
  "KUBERNETES_SERVICE_PORT",
  "KUBERNETES_SERVICE_HOST",
  "KUBERNETES_",
]

Fixes #187

[gabriel-samfira]

This looks great! Just one meaningful comment in regards to skipping any pattern that starts with GARM, to protect the variables that should be set by GARM itself when calling the external provider binary.