GHA/linux: fix mbedTLS cmake build

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 #15208

Closes #15238

.github/workflows/linux.yml

@@ -380,7 +380,8 @@ jobs:
           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 .