Support local `file://` release lists for offline version resolution

[kvendingoldo]

Is your feature request related to a problem? Please describe.
tenv already supports custom remotes and advanced remote configuration, which is very useful for companies that use artifact mirrors, private registries, or internal proxies.

However, some environments do not have reliable access to public APIs, GitHub release pages, HashiCorp release endpoints, or even internal HTTP mirrors. This is common for air-gapped installations, restricted CI runners, secure enterprise networks, and prebuilt container images.

In these cases, the required tool binary may already be available locally or through a controlled internal process, but tenv still needs to resolve a version constraint against the list of available versions. Today this flow generally assumes that the release list can be fetched from an HTTP/API source.

It would be useful if tenv could read the release/version list from a local file as well.

Describe the solution you'd like
Allow release list URLs to use the file:// scheme, so tenv can resolve versions from a local file without making a network request.

Example:

TFENV_LIST_URL=file:///opt/tenv/releases/terraform/index.json tenv tf install

[konan625]

I’ll take this one. The current download.JSON() only handles HTTP(S) URLs, so I’m going to extend it to support file:// as well. The idea is to follow the same pattern already used in GetPGPKey() (pkg/download/pgpkey.go), which reads from the local filesystem. Changes will be in pkg/download/download.go to add file:// support. Once done, it’ll work automatically for all retrievers (terraform, tofu, etc.) without any API changes.