CI: disable -O2 for Coverity after v9.1.0429

CI: disable -O2 for Coverity after v9.1.0429

gcc -O2 outputs this warning and turns it into an error when running
Coverity action:

```
eval.c: In function ‘echo_string_core’:
cc1: warning: function may return address of local variable [-Wreturn-local-addr]
eval.c:6495:12: note: declared here
 6495 |     char_u buf[MAX_FUNC_NAME_LEN];
      |            ^~~
```

This seems to be a false positive, so disable -O2 for Coverity
specifically.

Signed-off-by: Christian Brabandt <cb@256bit.org>