ci: build the package for Ubuntu 26.04

[lvaroqui]

Build the valgrind-codspeed package for Ubuntu 26.04, so the CodSpeed runner can install it there instead of asking for a manual installation or building valgrind from source.

• Release: the release matrix gains Ubuntu 26.04 for amd64 and arm64, so the next release publishes valgrind_<version>_ubuntu-26.04_{amd64,arm64}.deb next to the 22.04 and 24.04 packages. The arm64 package builds on the GitHub-hosted ubuntu-26.04-arm runner.
• CI: the Callgrind tests also run on ubuntu-26.04.
• Local checks: two Justfile recipes check a change or a package on an Ubuntu release other than the host's, in a throwaway container built from the tracked files of the working tree:
  • just docker-deb <ubuntu version> builds the .deb as the release workflow does (unsigned), installs it, runs Callgrind on it, and writes it to dist/ under its release asset name;
  • just docker-test <ubuntu version> runs the Callgrind tests as the CI does.

Verified on amd64 with these recipes: on 26.04 the Callgrind tests pass (24 tests, 0 failures), and the package builds, installs with its libc6-dbg dependency and runs Callgrind. The CodSpeed runner's executor test suite also passes on 26.04 against that package (CODSPEED_VALGRIND_DEB in the runner's just test-integ). The arm64 build is not verified yet: it first runs on the ubuntu-26.04-arm runner at release time.

Refs COD-3305, which stays open until the runner pins the 26.04 package from the next release.

[codspeed]

Merging this PR will not alter performance

✅ 84 untouched benchmarks
⏩ 60 skipped benchmarks¹

---

_{Comparing cod-3305-ubuntu-2604-package (12daf6d) with master (60a2e5f)}

Footnotes

1. 60 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[greptile-apps]

Confidence Score: 5/5

[High risk] Adds Ubuntu 26.04 to build and release pipelines.

The PR appears safe to merge; no actionable new issues were found.

What we checked:

• Existing package names still match: The release keeps the same naming pattern and adds new Ubuntu version entries. Existing package URLs remain unchanged.
• Package installs required debug symbols: The package depends on libc6-dbg. Docker installs the package through apt-get before running Callgrind, so that dependency is included.

Summary

Adds Ubuntu 26.04 to Callgrind CI and to release builds for amd64 and arm64.

• Adds just docker-deb to build, install, and smoke-test an unsigned package in Docker.
• Exports the package to ignored dist/ output under its release asset name.
• Fixes the earlier fresh-clone concern by initializing submodules before creating the archive.
• Keeps the asset naming expected by the CodSpeed runner. Runner support for the new package remains a separate follow-up.

_{Reviews (2) · Last reviewed commit: "ci: build the package for Ubuntu 26.04" · Reviewed by Greptile}

[GuillaumeLagrange]

olgtm, small improvements possible

[GuillaumeLagrange]

Lacks a bit of the flair that an inline docker exec script had, but overall this is slightly better.