fix(ci): upgrade runner to ubuntu-18.04

[jsjoeio]

Summary

This PR upgrades the Ubuntu runner 16.04 -> 18.04 for the package-linux-cross job in CI.

Context

4 days ago, CI was working as expected.

Yesterday, it stopped working.

After talking to @jawnsy, he shared with me that GitHub's infrastructure discontinued ubuntu-16.04: https://github.com/actions/virtual-environments/pull/3862/files on September 20th

Therefore, we have no other choice but to upgrade to 18.04.

Will this break anything? it shouldn't (famous last words)

Ubuntu 16.04 (Xenial Xerus) had glibc 2.23 and Ubuntu 18.04 (Bionic Beaver) has glibc 2.27, so I believe this may raise our minimum version requirement to 2.27: https://launchpad.net/ubuntu/+source/glibc
It looks like that version was released in early 2018, so that should be fine: https://sourceware.org/legacy-ml/libc-announce/2018/msg00000.html

— @jawnsy

Fixes N/A

[github-actions]

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