Make build require nightly Rust compiler

[maciejwitowski]

Running make build results in this error

error: the `-Z` flag is only accepted on the nightly channel of Cargo, but this is the `stable` channel

because it uses flag -Z from nightly channel

cargo build -Z unstable-options --release --out-dir ./bin -p ipc-cli

This may be a leftover from when we changed from using nightly to stable.