[spirv-headers] no absolute paths

[autoantwort]

For #20469

[github-actions]

You have modified or added at least one vcpkg.json where you should check the license field.

If you feel able to do so, please consider adding a "license" field to the following files:

• ports/spirv-headers/vcpkg.json

Valid values for the license field can be found in the documentation

[BillyONeal]

Thanks!