ci: Move --enable-c++20 from "tidy" task back to "ASan..." one

This change reverts cc7335edc8 partially.

C++20 has introduced some new headers, and it is premature to consider
them when using the IWYU tool.

ci/test/00_setup_env_native_tidy.sh

@@ -15,5 +15,5 @@ export RUN_FUNCTIONAL_TESTS=false
 export RUN_FUZZ_TESTS=false
 export RUN_TIDY=true
 export GOAL="install"
-export BITCOIN_CONFIG="CC=clang CXX=clang++ --enable-c++20 --with-incompatible-bdb --disable-hardening CFLAGS='-O0 -g0' CXXFLAGS='-O0 -g0'"
+export BITCOIN_CONFIG="CC=clang CXX=clang++ --with-incompatible-bdb --disable-hardening CFLAGS='-O0 -g0' CXXFLAGS='-O0 -g0'"
 export CCACHE_SIZE=200M