ci: fix coverage and valgrind jobs, skip fork tests under valgrind

- Install lcov in coverage job (not in fastsync-ci:v7)
- Use lcov 2.x compatible flags (--branch-coverage instead of --rc)
- Remove unused xxhash exclude pattern that lcov 2.x rejects
- Install valgrind in valgrind job
- Skip fork-based file tests under valgrind (pipe timing issues)
- Add FASTSYNC_UNDER_VALGRIND env var for test skip detection
- Move cleanup before assertions in fork tests to prevent leaks
- Add coverage.info to .gitignore

.gitignore

@@ -3,3 +3,4 @@ data_copied
 test_data/
 __pycache__/
 build-asan
+coverage.info