Update CONTRIBUTING.md to fix env activation instructions

[nthiery]

No description provided.

[nthiery]

Not absolutely sure of what I am doing here, but it seems to be working for me :-)

[bollwyvl]

As Mambaforge does come with conda, it will also have activate on $PATH, to be used by source or ..

As I work on... a lot of different (base) envs, I don't generally install the shell activation scripts. Hence (mamba|conda|micromamba|pixi) activate don't even work for me...

[nthiery]

As Mambaforge does come with conda, it will also have activate on $PATH, to be used by source or ..

Ah, I see, activate is in $PATH if I am in my base conda environment. But not necessarily in other
environments.

As I work on... a lot of different (base) envs, I don't generally install the shell activation scripts. Hence (mamba|conda|micromamba|pixi) activate don't even work for me...

Makes sense; though since the previous command line use a direct call to mamba,
presumably this should work for the second command line.

Feel free to close. A variant being to edit the line above as: "Start with mambaforge, in your base environment".

Cheers,