Fix build-reports.csv only containing a single line

[keysmashes]

Fixes #104.

Thanks to @Taneb for providing me with the changes to make here (once again, I really do not know Haskell).

[mweinelt]

Wow, first new appends since March 2025 here, thanks!

[maralorn]

Thank you so much @keysmashes for getting the ball rolling here.

In the end I decided to go with a different solution.

1. Using lazy IO at all was the mistake here so I didn’t want to rely on the forcing behavior and instead switched to a strict readFile. (Even with your fix I noticed weird side effects where e.g. reading an unparseable file would be closed slower.
2. According to the documentation of filelock one should not open the locked file. (Apparently it only matters on windows, but better safe than sorry.)

I will merge my solution and do a release soon.