feat: extract envbuilder binary from builder image (#13)

- Adds workspace_folder parameter.
- Adds functionality to extract the envbuilder binary from the builder image.
- Drive-by: updates provider address.

main.go

@@ -39,7 +39,7 @@ func main() {
 		// TODO: Update this string with the published name of your provider.
 		// Also update the tfplugindocs generate command to either remove the
 		// -provider-name flag or set its value to the updated provider name.
-		Address: "registry.terraform.io/hashicorp/envbuilder",
+		Address: "registry.terraform.io/coder/envbuilder",
 		Debug:   debug,
 	}