Update hermit cli version from Renovate bot

[OmarTawfik]

Follow up from #342

The current Renovate plugin supports updating versions of the binaries, but not the version of the hermit binary itself.

This is usually specified in the bin/hermit file, and it defaults to using the stable channel:

export HERMIT_DIST_URL="${HERMIT_DIST_URL:-https://github.com/cashapp/hermit/releases/download/stable}"

This means projects using Hermit can and will be broken if a bad version of Hermit gets released to Stable channel.

One solution would be to set a custom env var HERMIT_DIST_URL like https://github.com/cashapp/hermit/releases/download/v0.31.1. But that has two problems:

1. This variable needs to be set before Hermit is actually invoked and loaded, which means that we need an external way to set it, other than the supported/recommended way in hermit.hcl env property.
2. That will quickly go out of sync, and users need to update it manually regularly.

I suggest that if a repo locks the default installation URL in bin/hermit to specific versions, that Renovate bot updates it during regular updates. This means:

1. Users will stay up to date.
2. No need for an external (additional to Hermit) solution to manage environment variables before Hermit gets loaded.
3. This change is backwards compatible. users can still stay on stable channel if they want, and the bot won't change it if so.

[OmarTawfik]

@alecthomas actually, I tried to lock the hermit version downloaded in bin/hermit like this:

export HERMIT_DIST_URL="${HERMIT_DIST_URL:-https://github.com/cashapp/hermit/releases/download/v0.31.2}"

But this produces an error:

fatal:hermit: builtin:////hermit.hcl: no channel [email protected] found in channels (hermit@canary, hermit@stable) or versions (): unknown package

Is this a supported use case? Is there something else that can be modified to allow this?

[alecthomas]

Bit of an odd error, but no it is not supported. Hermit itself can currently only be updated from a channel.

[OmarTawfik]

Thanks for confirming.