Cabal ignores buildinfo in top-level of directory

The documentation states:

> If the package **root directory contains a file called package.buildinfo** after the configuration step, subsequent steps will read it to obtain additional settings for [build information](https://cabal.readthedocs.io/en/3.10/cabal-package.html#build- information) fields,to be merged with the ones given in the .cabal file. In particular, this file may be generated by the configure script mentioned above, allowing these settings to vary depending on the build environment.

However if you test this, then placing a .buildinfo file in the root of your project (and set the build type to `configure`) then it is ignored by cabal.

There is then some subsequent advice about using this with `install-includes`, which also doesn't work due to this bug.

This regression was introduced by #4874 which made Cabal look in the build directory for the .buildinfo rather than the current directory.

See tickets like #9401 work because they generate the `.buildinfo` file into the build directory (which is where configure is now run).

Sample project: https://gist.github.com/mpickering/bb13bae26146f0dfa39cb93241f1599c