last: add [[bin]] so the binary is named last

[MsfPablo]

Fixes #630.

src/uu/last/Cargo.toml declares [lib] but no [[bin]], so cargo falls back to the package name and emits target/release/uu_last. src/uu/last/src/main.rs already exists with the usual uucore::bin!(uu_last);, so only the manifest section was missing.

Added it in the same shape the other utilities use (renice, etc.). Checked the rest of src/uu/* — last was the only one affected.

Before:

$ ls target/debug | grep last
libuu_last.rlib
uu_last

After:

$ ls target/debug | grep last
last
libuu_last.rlib

Left the missing description field alone; that one is repo-wide and not what the issue is about.