BENCH/DEV: Add ``conda-build`` to ``environment.yml``

[Kai-Striega]

Is your feature request related to a problem? Please describe.

I'm unable to use the mamba command when running the benchmark suite. Please see the warning message in the additional context section.

Describe the solution you'd like.

Add conda-build to the environment.yml so that we can build the benchmarking suite without warnings

Describe alternatives you've considered.

Leave it as is, this could lead to issues later on. However, it looks like the tool falls back to conda, so it's not a significant issue at the moment.

Additional context (e.g. screenshots, GIFs)

(scipy-dev) kai@kai-MS-7D32:~/Projects/scipy$ python dev.py bench -t sparse_csgraph_dijkstra
💻  ninja -C /home/kai/Projects/scipy/build -j16
ninja: Entering directory `/home/kai/Projects/scipy/build'
[2/2] Generating scipy/generate-version with a custom command
Build OK
💻  meson install -C build --only-changed --tags runtime,python-runtime,tests,devel
Installing, see meson-install.log...
Installation OK
SciPy from development installed path at: /home/kai/Projects/scipy/build-install/lib/python3.12/site-packages
Running benchmarks for Scipy version 1.15.0.dev0+903.9406182 at /home/kai/Projects/scipy/build-install/lib/python3.12/site-packages/scipy
Couldn't load asv.plugins._mamba_helpers because
No module named 'libmambapy'
· Unknown environment type 'mamba'. Allowed values based on existing plugins are ['existing', 'conda', 'virtualenv']. If you are trying to use `mamba`, you may need to install `conda-build`.

[lucascolley]

Yeah, I wrote this error message for asv :) nice to see it has already been helpful!

Let's fix this as part of gh-21374.