Currently, the olddeps ci job specifies nixos-25.05 channel, but this is ignored in local ci runs, because the channel is set in .github/workflows/ci.yml not ci/scripts/run.sh.
Fix this and also rename CI environment variables to consistently use CI_ prefix
Currently, olddeps ci job specifies nixos-25.05 channel, but this is
ignored in local ci runs, because the NIXPKGS_CHANNEL is passed as the
cachiex nix path instead of being passed directly to nix develop.
Fix this and also rename the NIXPKGS_CHANNEL environment variable to
CI_NIXPKGS_CHANNEL because CI configuration variables are supposed to
have the CI_ prefix. The NIXPKGS_CHANNEL looks misleadingly like an
upstream configuration variable when it's just a local setting.
<!--e57a25ab6845829454e8d69fc972939a-->
The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.
<!--021abf342d371248e50ceaed478a90ca-->
See the guideline and AI policy for information on the review process. A summary of reviews will appear here.
<!--174a7506f384e20aa4161008e828411d-->
Reviewers, this pull request conflicts with the following ones:
If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.
<!--5faf32d7da4f0f540f40219e4f7537a3-->
looks like CI failed?
<!-- begin push-2 -->
Updated 723ba1c3934bd44e13644d14baf96280b7384b3e -> 7c73cceda9b4149b706dd667f70cbea381d6364a (pr/ci-channel.1 -> pr/ci-channel.2, compare)<!-- end --> to fix caching failure in gnu32 job https://github.com/bitcoin-core/libmultiprocess/actions/runs/32414275828/job/96571710982?pr=351