use consensus spec v1.5.0-alpha.0 test vectors; use Nim v2.0.4 specifically for 2.0.x

[tersec]

https://github.com/ethereum/consensus-specs/releases/tag/v1.5.0-alpha.0
https://github.com/ethereum/consensus-spec-tests/releases/tag/v1.5.0-alpha.0
towards https://notes.ethereum.org/@ethpandaops/pectra-devnet-0

Incorporate #6220 but without the upstream/ prefix which in this case was counterproductive

[tersec]

This time the v2.0.4 pin to work around nim-lang/Nim#23519 took:

Nim Compiler Version 2.0.4 [Linux: amd64]
Compiled at 2024-04-19
Copyright (c) 2006-2023 by Andreas Rumpf
git hash: b47747d31844c6bd9af4322efe55e24fefea544c
active boot switches: -d:release

x1B[92mBuilding:x1B[39m Nim compiler
Nim Compiler Version 2.0.4 [Windows: amd64]
Compiled at 2024-04-19
Copyright (c) 2006-2023 by Andreas Rumpf

git hash: b47747d31844c6bd9af4322efe55e24fefea544c
active boot switches: -d:release

Building: Nim compiler
Nim Compiler Version 2.0.4 [MacOSX: amd64]
Compiled at 2024-04-19
Copyright (c) 2006-2023 by Andreas Rumpf

git hash: b47747d31844c6bd9af4322efe55e24fefea544c
active boot switches: -d:release