Hello, Ben,
I suggest you to try MSYS2, which has an instaler. First, you install it (e.g. in C:\msys2). Then, run pacman -S mingw-w64-x86_64-toolchain in its command line interface to install MinGW64 in it. After that, just zip the MSYS2 installation folder and unpack it where you like. Finally, you likely need to configure CI/CD script to put the toolchain executables and libraries in PATH (D:\mySoftwaresPortables\msys64\mingw64\bin and D:\mySoftwaresPortables\msys64\mingw64\lib), for example, supposing you unzipped the MSYS2 installation directory to D:\mySoftwaresPortables\. That worked for me. That way you can have a working MinGW64 in your CI/CD environment without the need for a system-wide installation.
best,
PC