Install cmake on the cmake job

[thk123]

The cmake code build job has never succeeded. Originally to do with permissions, current failure appears to be because cmake isn't installed.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• [na] The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• [na] Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[thk123]

This causes the cbmc-linux-cmake-gcc build pass: https://us-east-1.console.aws.amazon.com/codesuite/codebuild/projects/cbmc-linux-cmake-gcc/build/cbmc-linux-cmake-gcc%3A484b61f7-3be1-421e-8ec1-d4da4e01938f/log?region=us-east-1

Not sure why the status isn't reported back here. Also I had to manually trigger it. I've asked @peterschrammel to see if something needs configuring still on the AWS side.

[hannes-steffenhagen-diffblue]

I'd hold off on merging it until we're sure the job actually runs now.

[thk123]

@hannes-steffenhagen-diffblue Peter said he'd fixed it - I've force pushed so hopefully will trigger it this time.