ci: avoid using -Werror for older compilers

Don't enable `-Werror` (in the CI) for compilers at least older than
our current release compiler (GCC 10). It provides little-to-no value,
other than turning compiler bugs & false positives into build failures,
and we aren't going to mutate perfectly fine/working code, for the sake
of avoid a warning that shouldn't even exist.

I also do not see the point of playing whack-a-mole and turning off various
warnings/trying to further work around the broken compiler, just to
acheive warningless builds for the sake of warningless builds.

One anecdote from "How SQLite Is Tested":
> Static analysis has found a few bugs in SQLite, but those are the
> exceptions. More bugs have been introduced into SQLite while trying
> to get it to compile without warnings than have been found by static
> analysis.

https://www.sqlite.org/testing.html.

ci/test/00_setup_env_i686_centos.sh

@@ -12,6 +12,7 @@ export CI_IMAGE_NAME_TAG=quay.io/centos/centos:stream8
 export CI_BASE_PACKAGES="gcc-c++ glibc-devel.x86_64 libstdc++-devel.x86_64 glibc-devel.i686 libstdc++-devel.i686 ccache libtool make git python38 python38-pip which patch lbzip2 xz procps-ng dash rsync coreutils bison"
 export PIP_PACKAGES="pyzmq"
 export GOAL="install"
+export NO_WERROR=1 # GCC 8
 export BITCOIN_CONFIG="--enable-zmq --with-gui=qt5 --enable-reduce-exports"
 export CONFIG_SHELL="/bin/dash"
 export TEST_RUNNER_ENV="LC_ALL=en_US.UTF-8"

ci/test/00_setup_env_native_nowallet_libbitcoinkernel.sh

@@ -13,4 +13,5 @@ export PACKAGES="-t buster-backports python3-zmq clang-8 llvm-8 libc++abi-8-dev
 export APPEND_APT_SOURCES_LIST="deb http://deb.debian.org/debian buster-backports main"
 export DEP_OPTS="NO_WALLET=1 CC=clang-8 CXX='clang++-8 -stdlib=libc++'"
 export GOAL="install"
+export NO_WERROR=1
 export BITCOIN_CONFIG="--enable-reduce-exports CC=clang-8 CXX='clang++-8 -stdlib=libc++' --enable-experimental-util-chainstate --with-experimental-kernel-lib --enable-shared"

ci/test/00_setup_env_native_qt5.sh

@@ -15,6 +15,7 @@ export TEST_RUNNER_EXTRA="--previous-releases --coverage --extended --exclude fe
 export RUN_UNIT_TESTS_SEQUENTIAL="true"
 export RUN_UNIT_TESTS="false"
 export GOAL="install"
+export NO_WERROR=1
 export DOWNLOAD_PREVIOUS_RELEASES="true"
 export BITCOIN_CONFIG="--enable-zmq --with-libs=no --with-gui=qt5 --enable-reduce-exports \
 --enable-debug CFLAGS=\"-g0 -O2 -funsigned-char\" CXXFLAGS=\"-g0 -O2 -funsigned-char\" CC=gcc-8 CXX=g++-8"