Option to only have echidna installed via action

It would be nice to have the ability to only install echidna to then be able to use/call it via other scripts, the same that [foundry-toolchain](https://github.com/foundry-rs/foundry-toolchain) allows it. In my case I'm trying to run a python script that executes a bunch of tests/challenges sequentially using echidna and foundry and measure performance (test outcomes, execution time & perhaps more later).

Start by inspecting the action's workflow and entry-point configuration to see how Echidna is currently installed. Compare its inputs and behavior with foundry-toolchain, then verify that a workflow can install Echidna without running it and that a later Python or shell step can invoke it.

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