feat: add support for setting build environment variables

[grumpydude]

This adds the capability to pass build time environment variables such as GOOGLE_FUNCTION_SOURCE.

[comment-bot-dev]

Thanks for the PR! 🚀
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• ⚠️check_terraform
  Failed Terraform check. More details below.

Running terraform fmt
Running terraform validate
terraform_validate . 
╷
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Error: 
Unsupported argument
│

│

 on main.tf line 89, in module "main":
│

 89:   
build_environment_variables
       = var.build_environment_variables
│

│

An argument named "build_environment_variables" is not expected here.
╵
terraform_validate ./examples/logs-slack-alerts 
╷
│

Error: 
Unsupported argument
│

│

 on ../../main.tf line 89, in module "main":
│

 89:   
build_environment_variables
       = var.build_environment_variables
│

│

An argument named "build_environment_variables" is not expected here.
╵
terraform_validate ./examples/pubsub_scheduled 
╷
│

Error: 
Unsupported argument
│

│

 on ../../main.tf line 89, in module "main":
│

 89:   
build_environment_variables
       = var.build_environment_variables
│

│

An argument named "build_environment_variables" is not expected here.
╵
terraform_validate ./examples/pubsub_scheduled_multiple 
╷
│

Error: 
Unsupported argument
│

│

 on ../../main.tf line 89, in module "main":
│

 89:   
build_environment_variables
       = var.build_environment_variables
│

│

An argument named "build_environment_variables" is not expected here.
╵
terraform_validate ./modules/project_cleanup 
╷
│

Error: 
Unsupported argument
│

│

 on ../../main.tf line 89, in module "main":
│

 89:   
build_environment_variables
       = var.build_environment_variables
│

│

An argument named "build_environment_variables" is not expected here.
╵
terraform_validate ./test/fixtures/pubsub_scheduled 
╷
│

Error: 
Unsupported argument
│

│

 on ../../../main.tf line 89, in module "main":
│

 89:   
build_environment_variables
       = var.build_environment_variables
│

│

An argument named "build_environment_variables" is not expected here.
╵
terraform_validate ./test/setup 
Success!
The configuration is valid.