helmfile: fix dockerfile issues

[jklinect]

Tried assembling/using this container in a Cloud Build recently, it didn't work as expected when running it:

Fetching cluster endpoint and auth data.
kubeconfig entry generated for acme-kube-cluster.
/builder/helmfile/helmfile: line 1: Not: command not found

(/builder/helmfile/helmfile is a 9-byte file with the contents Not Found)

These changes restore it to functional order:

• references the correct github repo/URI for the download (helmfile/helmfile)
• stages the tarball/extracts the helmfile properly
• added -f to the curl call so that, if the URI moves on the helmfile tarball again, it'll fail the container build
• update the default versions of both helm/helmfile in the yaml to the latest & greatest

The container both builds and runs as expected now:

Fetching cluster endpoint and auth data.
kubeconfig entry generated for acme-kube-cluster.
Adding repo acme-project https://acme.project.tld
"acme-project" has been added to your repositories
...

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