Fix OpenMPI cache miss in CI workflows

[Geethapranay1]

This pr fixes CI failures on branch or pr runs by building OpenMPI when the cache is missing, instead of assuming ~/openmpi always exists

closes #259

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[Geethapranay1]

@IshaanDesai PTAL

https://github.com/precice/micro-manager/actions/runs/25684751288/job/75405373475