HELM_DIFF_USE_UPGRADE_DRY_RUN=true doesn't work for new install

[mumoshu]

I was running a few helm-diff commands against my local kind cluster with Helm 3.7.2 and the edge version of helm-diff built from #364.

It worked great for upgrades, but not for new installs. When it's a diff for a new install it failed with:

HELM_DIFF_USE_UPGRADE_DRY_RUN=true HELM_NAMESPACE=default HELM_BIN=helm372   ./helm-diff upgrade my3 stable/mysql
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        Release was not present in Helm.  Include the `--allow-unreleased` to perform diff without exiting in error.

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Error: exit status 1: Error: release: not found

Running it with --allow-unreleased (or --install which is a preferred alias of --allow-unreleased) as advised in the message didn't prevent the error.

After a bit of digging, this turned out to be a bug in helm-diff, which doesn't pass --install to the helm-upgrade-dry-run.