fixing versions for dependencies

As we do releases more often now, I think we need to be more careful with what we install in our conda environment. For example, with a recent issue of Astra toolbox moving to conda-forge and dropping support of the older cudatoolkit versions there was an error in the environment installed by default.

To be on the safe side, we probably need to fix all the packages in our environment so that the environment will be reproduced exactly for the future releases? It is convenient that we install Htttomo + libraries through `pip` so that the packages in already created environment will not be changed. We should generate a list of `explicit` packages from the working environment and reuse it until we _actually_ need to update some of the packages in that environment.
And the main [environment](https://github.com/DiamondLightSource/httomo/blob/main/conda/environment.yml) file should also have the main packages versions fixed as well, e.g.:
```
- astra-toolbox == 2.1.1
- python == 1.25
- numpy == 1.25
- cupy==12.3.0
- cudatoolkit==11.2
```

I hope that this will reduce the chance of breaking things with our local installations especially.