Correctly build native_mixed on bionic and focal.

If `native_mixed` is part of the original target, the "include" with the
`image_variant` will overwrite the combination.

If we remove `native_mixed`, both combinations are added.

See https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idstrategymatrixinclude

.github/workflows/releaseNigthly.yml

@@ -15,7 +15,6 @@ jobs:
         target:
           - native_static
           - native_dyn
-          - native_mixed
           - wasm
           - armv6_static
           - armv6_mixed