Helmfile: Add possibility to set helm executable in helm3 mode

[pavdmyt]

I'm using both helm2 and helm3 for managing different environments. For production clusters helm2 is used and we've started experimenting with helm3 for our new clusters.

At my laptop I'm having helm pointing to _helm2_ binary and helm3 pointing to _helm3_ binary. Running helmfile with HELMFILE_HELM3="1", there are following errors:

$ helmfile -f cert-manager.yaml diff                                                                                                              
Adding repo jetstack https://charts.jetstack.io
"jetstack" has been added to your repositories

Updating repo
Hang tight while we grab the latest from your chart repositories...
...Successfully got an update from the "jetstack" chart repository
...Successfully got an update from the "stable" chart repository
Update Complete.

Comparing release=cert-manager, chart=jetstack/cert-manager
in ./cert-manager.yaml: 2 errors:
err 0: failed processing release cert-manager: helm exited with status 1:
  Error: could not find tiller

Under the hood helmfile executes commands using helm and it seems there is no way to specify which binary to use. Is it possible to add e.g. env variable to specify this?

I've fixed the issue by renaming helm binary to helm2 and creating symlink: helm -> helm3. But it's not very convenient.

[mumoshu]

@pavdmyt Hey! Are you looking for the --helm-binary flag that exists today, or
anything else?

[pavdmyt]

@mumoshu this is exactly what I need 😃

I've been searching through the codebase for appropriate env var not thinking about the flag.

[mumoshu]

We need to do better job surfacing this feature. Anyways, thanks for using Helmfile ☺️