ci: fix perftest uploading

I have noticed that we were failing to upload Performance tests records, the reason was a wrong usage of the ENV inside a cmake conditional. I am puzzled how this was able to work in a few commits in the past. This might have been a flaky behaviour of cmake.

Here is a snippet explaining how to test non empty env variables in cmake conditonals: https://godbolt.org/z/9a1rfzEh5