github: Do not test meson on Ubuntu 18.04
We are bumping Meson version to 0.53, which Ubuntu 18.04 LTS doesn't
have.  Remove the combination of meson on Ubuntu 18.04 from the
matrix.
@@ -15,6 +15,9 @@ jobs:
- meson
- cmake
- waf
+ exclude:
+ - os: ubuntu-18.04
+ buildsystem: meson
runs-on: ${{ matrix.os }}
steps:
- name: Setup packages on Linux
