Try to get the coverity scan results back under the size limit

Apparently our Coverity Scan file we try to upload is too large for the
server to handle, so it isn't working. Try switching to a release build
with debug symbols to see if that reduces the size.

.gitlab/coverity.yml

@@ -56,7 +56,7 @@ Coverity:
   script:
     - *coverity_cache_prep
     - cmake
-      -DCMAKE_BUILD_TYPE=Debug
+      -DCMAKE_BUILD_TYPE=RelWithDebInfo
       -DKICAD_STDLIB_LIGHT_DEBUG=ON
       -DKICAD_SCRIPTING_WXPYTHON=ON
     - coverity/bin/cov-build --dir cov-int make -j8