release-linux: pin runner to ubuntu-22.04 and guard the glibc baseline

[faisalahmedsifat]

Summary

Built on ubuntu-latest (24.04, glibc 2.39), the release binary refuses to start on 22.04-based distros (Pop!_OS 22.04, Mint 21): the loader fails with GLIBC_2.38 / GLIBC_2.39 not found even though the package installs fine. glibc is backward-compatible only, so build on the oldest supported Ubuntu (22.04, glibc 2.35) to stay runnable everywhere newer.

Jammy's default gcc-11 also cannot compile numkong's AVX512-FP16 kernels, so point CC/CXX at gcc-12. A post-build objdump check fails the job if the binary ever requires newer than GLIBC_2.35 again.

Changes

• Pin runs-on to ubuntu-22.04
• Install and use gcc-12 / g++-12
• Add a glibc baseline guard after the build

Test plan

• Confirm the Linux release workflow still packages successfully on the pinned runner
• Confirm the produced binary does not require symbols newer than GLIBC_2.35

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