cmake: Fix potential configure error when setting up buildnumber

[WarmUpTill]

Description

If the CI environment variable is defined but GITHUB_RUN_ID is not this will result in a broken if statement.
This can apparently be the case for some Debian build pipelines.

This problem was originally reported here, while packaging obs-advanced-scene-switcher:
WarmUpTill/SceneSwitcher#1090

Motivation and Context

Fixes potential configure error.

How Has This Been Tested?

Tested by manually defining only the CI variable.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[PatTheMav]

If it breaks because GITHUB_RUN_ID is not set, then the code (as it's currently implemented) works exactly as it should. Wrapping it in double quotes (to make it empty strings) defeats the purpose.

I think it would be better to first only check for the CI environment variable and then add a check for GITHUB_RUN_ID or GITLAB_RUN_ID and then use whatever is available to generate a build ID.

Because the code right now makes the assumption that CI would only ever be GitHub Actions, but I don't think it necessarily needs to be limited to that in the CMake files.

[WarmUpTill]

Makes sense.
I have updated the PR.

[PatTheMav]

Could you update the commit title to make it more explicit that this adds support for using GitLab run IDs as build numbers (because that's the precise scope of the change now)?

[WarmUpTill]

Done :)