[Compiler download] Allow multiple parallel downloads of different compilers

[ChristopherDedominici]

Previously, downloadCompiler() used a single shared MultiProcessMutex("compiler-download"), a file-based lock at, for example, /tmp/compiler-download.txt. This meant that across all parallel test workers, only one could download or even check if a compiler exists at a time. Workers needing different compiler versions would still block each other.

The fix makes the mutex version-specific (compiler-download-${version}), so workers downloading different versions proceed in parallel. Same-version downloads are still correctly serialized.

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[schaable]

LGTM. The mutex logic already has its own test in v-next/hardhat/test/internal/builtin-plugins/solidity/build-system/compiler/downloader.ts, which should cover this change.

[kanej]

This has been released as part of hardhat@3.1.10: https://github.com/NomicFoundation/hardhat/releases/tag/hardhat%403.1.10