CMake builds mbedTLS in Debug mode by default, which was the reason
for these consistent test failures:
```
FAIL 1631: 'FTP through HTTPS-proxy' FTP, HTTPS-proxy
FAIL 1632: 'FTP through HTTPS-proxy, with connection reuse' FTP, HTTPS-proxy
```
Sometimes also:
```
FAIL 303: 'HTTPS with 8 secs timeout' HTTPS, HTTP GET, timeout, FAILURE
```
https://github.com/curl/curl/actions/runs/11260616621/job/31313234198
Fix it by building in `RelWithDebInfo` mode, matching the bare
`Makefile` builds used earlier. (`Release` mode also works.)
Cache sizes:
- Makefile: 10MB
- CMake Release: 1MB
- CMake RelWithDebInfo: 2.5MB
Ref: #15215
Follow-up to e377c91766#15208Closes#15238