ci: Avoid -Wno-error=maybe-uninitialized false positives

Avoid false positive maybe-uninitialized errors in native_previous_releases
CI job.

Fix was suggested by maflcko in
bitcoin#25665 (comment)

CI errors look like:
https://github.com/bitcoin/bitcoin/actions/runs/21052613538/job/60542048502?pr=29700

/usr/include/c++/12/variant:1788:15: error: ‘((const std::__detail::__variant::_Variant_storage<true, VerifySuccess, kernel::Interrupted, SkippedL3Checks, SkippedMissingBlocks>::__index_type*)((char*)&verify_result + offsetof(util::Result<std::variant<VerifySuccess, kernel::Interrupted, SkippedL3Checks, SkippedMissingBlocks>, void, kernel::FlushStatus, util::Messages>,util::Result<std::variant<VerifySuccess, kernel::Interrupted, SkippedL3Checks, SkippedMissingBlocks>, void, kernel::FlushStatus, util::Messages>::<unnamed>.util::detail::SuccessHolder<std::variant<VerifySuccess, kernel::Interrupted, SkippedL3Checks, SkippedMissingBlocks>, void, kernel::FlushStatus, util::Messages>::<unnamed>)))[1]’ may be used uninitialized [-Werror=maybe-uninitialized]
 1788 |               switch (__v0.index())
      |               ^~~~~~
/home/admin/actions-runner/_work/_temp/src/node/chainstate.cpp: In function ‘kernel::FlushResult<std::variant<std::monostate, kernel::Interrupted>, node::ChainstateLoadError> node::VerifyLoadedChainstate(ChainstateManager&, const ChainstateLoadOptions&)’:
/home/admin/actions-runner/_work/_temp/src/node/chainstate.cpp:281:18: note: ‘((const std::__detail::__variant::_Variant_storage<true, VerifySuccess, kernel::Interrupted, SkippedL3Checks, SkippedMissingBlocks>::__index_type*)((char*)&verify_result + offsetof(util::Result<std::variant<VerifySuccess, kernel::Interrupted, SkippedL3Checks, SkippedMissingBlocks>, void, kernel::FlushStatus, util::Messages>,util::Result<std::variant<VerifySuccess, kernel::Interrupted, SkippedL3Checks, SkippedMissingBlocks>, void, kernel::FlushStatus, util::Messages>::<unnamed>.util::detail::SuccessHolder<std::variant<VerifySuccess, kernel::Interrupted, SkippedL3Checks, SkippedMissingBlocks>, void, kernel::FlushStatus, util::Messages>::<unnamed>)))[1]’ was declared here
  281 |             auto verify_result{CVerifyDB(chainman.GetNotifications()).VerifyDB(
      |                  ^~~~~~~~~~~~~

Author: ryanofsky

ci/test/00_setup_env_native_previous_releases.sh

@@ -22,7 +22,14 @@ export BITCOIN_CONFIG="\
  -DCMAKE_BUILD_TYPE=Debug \
  -DCMAKE_C_FLAGS='-funsigned-char -Werror' \
  -DCMAKE_C_FLAGS_DEBUG='-g2 -O2' \
+<<<<<<< HEAD
  -DCMAKE_CXX_FLAGS='-funsigned-char -Werror' \
+||||||| parent of 326c23ab46e (ci: Avoid -Wno-error=maybe-uninitialized false positives)
+ -DCMAKE_CXX_FLAGS='-funsigned-char' \
+=======
+ -DCMAKE_CXX_FLAGS='-funsigned-char' \
+ -DCMAKE_CXX_FLAGS='-Wno-error=maybe-uninitialized' \
+>>>>>>> 326c23ab46e (ci: Avoid -Wno-error=maybe-uninitialized false positives)
  -DCMAKE_CXX_FLAGS_DEBUG='-g2 -O2' \
  -DAPPEND_CPPFLAGS='-DBOOST_MULTI_INDEX_ENABLE_SAFE_MODE' \
 "