Make envsubst callable (add main)

[guettli]

Thank you very much for incorporating my PR to the upstream (unmaintained) envsubst: drone/envsubst#34

I notice that you do not provide a main package, so the package is not callable as a binary.

What about adding a main, so that we can install a binary via go install?

I could provide a PR, but I guess you fill do it fast, so feel free to do it, if you think it is useful.

[stefanprodan]

You can use flux envsubst, the Flux CLI is available for all platforms and as a container image, see https://fluxcd.io/flux/installation/#install-the-flux-cli

[guettli]

@stefanprodan Cluster API uses the unmaintained envsubst a lot. I think the capi maintainers don't want depend on the flux command line tool.

[stefanprodan]

The fluxcd/pkg repo is for Go packages, having some command/exec project in here is not feasible. You could create a repo in Cluster API and implement there a standalone CLI that import fluxcd/pkg/envsubst.

[guettli]

@stefanprodan thank you for your reply. Yes, your suggestion is a good solution.

[guettli]

I close the fluxCD issue, continuing here: kubernetes-sigs/cluster-api#8325