In order to support some use-cases around caching Terraform modules and provisioners, we need to allow setting some environment variablesspecifically for certain Terraform invocations.

One such example would be settingHTTP_PROXY /HTTPS_PROXY to a caching proxy duringterraform init, so that cached versions of providers and modules can be retrieved from the proxy instead of upstream.

ExposeCODER_PROVISIONER_TERRAFORM_{INIT,PLAN,APPLY}_ENV on bothcoderd andprovisionerd that will allow passing an array of environment variables intoterraform.provisionEnv.

For the above example, an administrator could setCODER_PROVISIONER_TERRAFORM_INIT_ENV_0=HTTP_PROXY=http://proxy.corp.int:8080.

NOTE: It shouldnot allow overriding any environment variable starting with the prefixCODER_.