specifying go build main package location

my main.go file is located in cmd/main.go
apparently the flyctl returns an error stating that it could not find the main file with the package main
I may want to specify something like this go build -o /layers/paketo-buildpacks_go-build/targets/bin -buildmode pie -trimpath cmd/main.go
that’s the only thing that breaks the build.

I don’t know much about the fly.toml, perhaps there are configurations/documentations about it, then let me know
fyi: default auto-generated fly.toml is used

Paketo Buildpack for Go Build 2.0.3
  Executing build process
    Running 'go build -o /layers/paketo-buildpacks_go-build/targets/bin -buildmode pie -trimpath .'
failed to execute 'go build': exit status 1
      Failed after 72ms
        go: no main packages to build