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