Support `env` template function in arguments

[szh]

According to the README, all Sprig template functions are supported in helm arguments. However, it seems that the env function is not supported, presumably because Helm doesn't support it natively. It would be helpful in the case of an argument that must be generated on the fly.

For example, I'm trying to create recipe for conjur-oss and it requires an argument dataKey which is intended to be generated just before the helm install command is called. I wanted to add the command to generate it into a pre_install step, but then there's no apparent way to pass that to the helm argument.