github/workflows: disable seccomp for linux native CI

This CI builder bases on openSUSE Tumbleweed, and recently had
its glibc updated. This led to new syscalls such as 'clone3' not
being allowed through the security layer.

Can be reverted after Github Actions updates their security policy.

actions/runner-images#3812

.github/workflows/build.yml

@@ -72,6 +72,10 @@ jobs:
     runs-on: "ubuntu-20.04"
     container:
       image: "registry.cirno.systems/kiwi/containers/mpv-ci:stable-deps"
+      # Disable seccomp until a container manager in GitHub recognizes
+      # clone3() syscall,
+      # <https://github.com/actions/virtual-environments/issues/3812>.
+      options: --security-opt seccomp=unconfined
       env:
         CC: "${{ matrix.cc }}"
     strategy: