Use custom --buildstate-path for lektor builds

The buildstate file Lektor creates when building the site is placed inside a .lektor directory inside the output path. However, this buildstate is useless in artifacts and can become quite large (1.7G for the blog project). The --buildstate-path switch allows us to use a different path to store it. Using cache for this makes sense as subsequent builds may refer to it, although ideally we'd also have the actual build artifacts and not just the buildstate.

This replaces the .cache/lektor/builds/ cache path: it does not seem to be actually used in the build process as shown by this error message in the CI builds of Lektor:

WARNING: .cache/lektor/builds/: no matching files