Prepend drive to jenkins workspace if missing on VM agents

[thomashampson]

Description of work.
The WORKSPACE variable used in the nightly pipeline Jenkinsfile is missing the C: on cloud VM Windows nodes. Here it is prepended if it's not there.

To test:

Verify that this job passed all build/test and package steps:
https://builds.mantidproject.org/job/build_packages_from_branch/249/

Fixes #35119

This does not require release notes because it's a change to our build config

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Reviewer

Please comment on the following (full description):

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards?
• Are the unit tests small and test the class in isolation?
• If there is GUI work does it follow the GUI standards?
• If there are changes in the release notes then do they describe the changes appropriately?
• Are the release notes saved in a separate file, using Issue or PR number for file name and in the correct location?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
• Do the changes handle unexpected situations, e.g. bad input?
• Has the relevant (user and developer) documentation been added/updated?

Does everything look good? Mark the review as Approve. A member of @mantidproject/gatekeepers will take care of it.

[MialLewis]

Code looks good, build package test has passed.