Force reconfigure in CI

The cached build.ninja may reference files that don't exist in the PR.
On a local build this tends to auto-fix itself as the build scripts detect
a quick failure and re-run the configure, but CI tends to be too slow.

https://github.com/ankitects/anki/pull/4364#issuecomment-3338026129

.buildkite/linux/entrypoint

@@ -16,6 +16,7 @@ if [ "$CLEAR_RUST" = "1" ]; then
     rm -rf $BUILD_ROOT/rust
 fi
 
+rm -f out/build.ninja
 ./ninja pylib qt check
 
 echo "--- Ensure libs importable"