Fix the path to VERSION.txt used in CMake.

[grynspan]

Fixes a bug introduced in #1320.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

Merging before CI finishes as none of it uses CMake anyway so this won't affect it.

[mhegazy]

thanks!