Fix library path in test build script

[blazej-jaworowski]

Currently nvim-oxi tests get rebuilt even when nothing was changed because of a bug in src/tests/build.rs.

Rust build scripts don't support "stuff" syntax, so path /some/absolute/path resolves to /working/directory/"/some/absolute/path", causing (in my case)

0.035248528s  INFO prepare_target{force=false package_id=eel-nvim v0.0.1 (/home/bjaworowski/projects/eel/nvim/eel-nvim) target="eel_nvim"}: cargo::core::compiler::fingerprint: stale: missing "/home/bjaworowski/projects/eel/nvim/eel-nvim/\"/home/bjaworowski/projects/eel/target/nvim_oxi_tests/eel-nvim/debug/libeel_nvim.so\""

and rebuilding of tests at every run.