ci: use archlinux:base-20210214.0.15477 as base for building

This gives us an arch base from before the glibc version that enforces
a newer glibc package that has a hard dependency on a newer linux
kernel.
2 jobs for master in 6 minutes and 22 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1177908
latest-lcms

00:06:22

passed #1177909
latest-nolcms

00:01:26