unpin mamba and conda in package-conda environment

[thomashampson]

Describe the outcome that is desired.
Mamba and Conda have been pinned in the package-conda environment to avoid two unrelated issues. Mamba was pinned here. Conda was pinned here. Ideally we wouldn't need to pin them.

Describe any solutions you are considering
Mamba, Conda, conda-build, (...?), shouldn't be installed outside of the base environment, so maybe we should refactor the scripts a bit to avoid doing this. The documentation states that installing them outside of the base env is unsupported.

Additional context
Reasons for pinning:

• mamba 1.5 -> 2.0 lost the --copy option for the create command, which stopped the standalone packaging environment creation. I tried replacing with --always-copy as advised. that worked, but there were further issues that I didn't have time to resolve. The missing option has been added into mamba 2.0.1 now.
• conda 24.7 -> 24.9.{0,1} for some reason has broken installing mantidworkbench on Windows, but only on the CI. It cannot run the post-link.bat script. See here for more info.