Fix windows post linking

[thomashampson]

Description of work

Something has changed between conda 24.7 and 24.9 to cause a failure when installing the mantidworkbench package on our Windows CI machines. It fails at the post-link.bat stage, which is required to set a Qt plugin path environment variable:

ERROR conda.core.link:_execute(951): An error occurred while installing package 'file:///C:/jenkins_workdir/workspace/release-next_nightly_deployment/conda-bld::mantidworkbench-6.10.20241002.1135-py310h424512f_0'.
Rolling back transaction: ...working... done
post-link script failed for package file:///C:/jenkins_workdir/workspace/release-next_nightly_deployment/conda-bld::mantidworkbench-6.10.20241002.1135-py310h424512f_0
location of failed script: C:\jenkins_workdir\workspace\release-next_nightly_deployment\source\installers\conda\win\_conda_env\Scripts\.mantidworkbench-post-link.bat
==> script messages <==
<None>
==> script output <==
stdout:
stderr: The system cannot find the path specified.

return code: 1

Pinning conda in the package-conda environment appears circumvent the problem. We do this for now to get the nightly running and investigate further later. For some reason I wasn't able to reproduce the problem locally but since there were no code changes that could cause this it is almost certainly the conda version change that has broken it.

There is no associated issue.

To test:

Verify the packages are build in this job:
https://builds.mantidproject.org/job/build_packages_from_branch/916/
Install the Windows conda package with:

mamba create -n workbench_windows_test
mamba activate workbench_windows_test
mamba install -c thomashampson/label/postlinkfix mantidworkbench

and verify that it launches with the workbench command.

This does not require release notes because it's a build script change

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[jclarkeSTFC]

I'll check the Windows conda install

[SilkeSchomann]

I can confirm that workbench starts as expected when following the test instructions.

Since the windows test failure was not caused by these changes I am happy to approve.

[jclarkeSTFC]

Conda install worked fine on my Windows machine.