Compile in release mode for at least one CI runner (#10591)

When we changed our default from RelWithDebInfo to Debug, we inadvertently ended
up with all CI building and running in Debug mode. Change at least one of them
back to Release to make sure we don't have any optimizations that cause funky
stuff.

I'm changing the Ubuntu CI image because it's hopefully the fastest (since rust
is relatively dog-slow to compile in release mode).

.github/workflows/main.yml

@@ -26,7 +26,7 @@ jobs:
     - name: cmake
       run: |
         mkdir build && cd build
-        cmake ..
+        cmake .. -DCMAKE_BUILD_TYPE=RelWithDebInfo
     - name: make
       run: |
         make VERBOSE=1