ci: key dialyzer cache by erlang/elixir version

[OmarBrbutovic]

What this PR does / why we need it:

Dialyzer was intermittently failing with Old PLT file when the cached PLT was restored after the Erlang/Elixir toolchain changed.

Update the backend dialyzer workflow to compute a cache prefix from the erlang and elixir entries in .tool-versions. Forcing a rebuild when the runtime that affects Dialyzer changes.

Additional documentation e.g. usage docs, diagrams, reviewer notes, etc.:

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[Dam-99]

i'm sure it was intentional faliures 👀

[OmarBrbutovic]

I’m sure those failures were intentional 👀

Poorly worded on my side. The goal is to avoid having to manually clear the cache every time something updates. With Renovate in place, that could happen quite frequently.

There are still a few edge cases we should explore, but we’ll expand on those once they come up again.

And, of course, naming and commit titles remain the hardest part of development.

[Dam-99]

@OmarBrbutovic don't worry, i was just messing around, i approve of these changes