Guard against compiler download race condition

This follows from our work on the solx downloader: https://github.com/NomicFoundation/hardhat/pull/8556

The core solc downloader has a similar issue. We test whether a downloader is cached locally by checking for file existence, however, a failure leads to a three step process:

1. download file
2. Verify checksum on file
3. chmod the file to allow execution

Only step 1 is required for other processes to see a cached compiler, even though it may not be verified nor executable.

We should resolve this by using the same approach as we used for solx, download to a tmp file location then atomically move it into place once verification and chmod have happened.