rust: save cargo and CARGO_HOME to variables

rust: save cargo and CARGO_HOME to variables

During configure, substitute the path of cargo, as well as the
value of CARGO_HOME as variables. This fixes the case where a
user might do:
  make
  sudo make install
Which will cause the cargo bits to be rebuilt, including
re-downloading external crates.

By saving these to variables we can be sure that the same
values are used during make install as were used during
make which prevents the Rust artifacts from being rebuild
during "sudo make install".