Fix b2 command on Windows (MinGW) CI

Author: Flamefire

.github/workflows/ci_tests.yml

@@ -103,7 +103,7 @@ jobs:
       shell: cmd
       run: |
         cd __buildBoost
-        ./b2 link=static toolset=$B2_TOOLSET address-model=64 -j$(nproc) --prefix="$BOOST_ROOT" install
+        .\b2 link=static toolset=$B2_TOOLSET address-model=64 --prefix="$BOOST_ROOT" install
         cd ..
         rm -r __buildBoost || true
     - name: Create standalone