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
e377c917664241d8cccf67316b96d59a280ad8e4 #15208
Closes #15238
git clone --quiet --depth=1 -b v${{ env.mbedtls-version }} https://github.com/Mbed-TLS/mbedtls
cd mbedtls
git submodule update --init
- cmake -B . -G Ninja -DENABLE_PROGRAMS=OFF -DENABLE_TESTING=OFF -DCMAKE_POSITION_INDEPENDENT_CODE=ON -DCMAKE_INSTALL_PREFIX=$HOME/mbedtls
+ cmake -B . -G Ninja -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_POSITION_INDEPENDENT_CODE=ON -DCMAKE_INSTALL_PREFIX=$HOME/mbedtls \
+ -DENABLE_PROGRAMS=OFF -DENABLE_TESTING=OFF
cmake --build .
cmake --install .