GHA: Fix CMake tests on MinGW

The runners have MinGW installed in 2 locations:
- C:\Program Files\Git\mingw64 uses msvcrt.dll and ucrtbase.dll
- C:\mingw64 uses only (the new) ucrtbase.dll

CMake is using the compiler from C:\mingw64 but at runtime it is still loading the
`libstdc++-6.dll` from the Git-MinGW as it is started from the Git bash.

This causes conflicts with functions compiled into the executable and those used by the runtime.
Most notably the string from `abi::__cxa_demangle` is allocated on a different heap
so freeing it with `free` called from the executable corrupts the heap leading to a SIGSEGV / Exit code 0xc0000374 in the tests.

B2 doesn't suffer from this as it modifies `$PATH` when running each test such that the right libraries are used.
Do it similarly in the GHA-CI config.

.github/workflows/ci.yml

@@ -226,7 +226,14 @@ jobs:
             extra_args+=(-G "${{matrix.generator}}")
           fi
           if [[ "${{matrix.generator}}" == "MinGW"* ]]; then
+            # Avoid "Fatal error: can't write 1214 bytes to section .text of [...] 'file too big' [...] too many sections"
             extra_args+=(-DCMAKE_CXX_FLAGS="-Wa,-mbig-obj")
+            # GHA has 2 MinGWs installed which must not be mixed,
+            # so make sure the executables pick up the libraries/DLLs matching the compiler
+            gcc_folder=$(dirname "$(which g++)")
+            prefix=${gcc_folder%/bin}
+            echo "GCC prefix: $prefix"
+            export PATH="$prefix/bin:$prefix/lib:$PATH"
           fi
           if [[ -n $B2_CXXFLAGS ]]; then
             extra_args+=(-DCMAKE_CXX_FLAGS="$B2_CXXFLAGS")
@@ -242,7 +249,7 @@ jobs:
           if [[ -n "$CXX_STD" ]]; then
             extra_args+=(-DCMAKE_CXX_STANDARD="$CXX_STD")
           fi
-          echo "Using extra args: ${extra_args[*]}"
+          set +x
           cmake .. -DCMAKE_BUILD_TYPE=Debug -DBoost_ROOT="$BOOST_ROOT/stage" "${extra_args[@]}" -DCMAKE_VERBOSE_MAKEFILE=ON -DBoost_DEBUG=ON -DBoost_VERBOSE=ON
           cmake --build . --config Debug --parallel 3
           ctest --output-on-failure --build-config Debug