Fix `dapp---use` and `solc---use` for flakes

[asymmetric]

Description

The main goal of this PR is to allow dapp --use and seth --use for flake users. This requires a series of internal changes:

• bumping the nix version (as the previously pinned one was pre-flakes), which in turn required
• bumping the nixpkgs version, which in turn required
• switching to ethereum/hevm (as there was no hope of getting the in-tree hevm to compile with a newer nixpkgs).

So to summarize:

• Fixes usage of dapptools with flakes
• Switches to ethereum/hevm (incorporating work from #968)
• Fixes a bug in seth---use where seth would be run twice
• Adds a devShell to flake
• Adds a CI job that runs nix build (in the future, it should supplant the existing one, and run nix flake check)
• And a few other minor tweaks

Checklist

• [x] tested locally
• [x] added automated tests
• [x] updated the docs
• [x] updated the changelog

closes #968

[d-xo]

-18,000 lines :exploding_head:

[asymmetric]

@d-xo I think this error (test_run_tx_source_fetching) is the one we talked about and that has to do with hevm.

Not entirely sure what the deal is with this one though (test_4byte) -- works on my machine.

[d-xo]

I think test_4byte is just flakey and can be disabled, it was also failing in this pr: https://github.com/dapphub/dapptools/pull/969

[d-xo]

looks like ci is still failing because of: https://github.com/ethereum/hevm/pull/198? this should be fixed in main

[asymmetric]

looks like ci is still failing because of: ethereum/hevm#198? this should be fixed in main

For the record, there's still a failing test, pending a fix on hevm.