Does not build on docs.rs

[freshtonic]

See the build logs here: https://docs.rs/crate/sqltk/0.1.0/builds/1573774

Hypothesis: the build process depends on cargo-expand being available.

We could make it so that build.rs tests for its presence and installs it when it detects that it is running in docs.rs. It should never be installed automatically for developers.