Add arch command for agnostic support of architecture in build command

[mcncl]

The current command is incorrect if the user isn't using Linux AMD 64 architecture. This command will use the output of arch and fill the dir.

❯ arch
arm64

go build -o terraform-provider-buildkite . && \                                
  mkdir -p tf-proj/terraform.d/plugins/registry.terraform.io/buildkite/buildkite/0.5.0/$(arch)/ && \
  mv terraform-provider-buildkite tf-proj/terraform.d/plugins/registry.terraform.io/buildkite/buildkite/0.5.0/$(arch)/

ls -al tf-proj/terraform.d/plugins/registry.terraform.io/buildkite/buildkite/0.5.0/      
drwxr-xr-x - ben 15 Mar 09:58 arm64