ci: deduplicate zig version

```
env:
  ZIG_VERSION: 0.14.0-dev.1550+4fba7336a
```

is present twice in our CI, one for each file.

Because we are using the master branch of the Zig language, this hash gets obsolete quite quickly and has to be changed frequently. 
It would be better to have it declared only a single time and shared between all the CI's workflow.
It would avoid forgetting editing one the files