Makefile references nonexistent file

### Issue
When attempting to install grip-tools with the provided makefile, it fails due to containing a reference to a non-existent file. Namely, `grip-tools-selfupdate.sh`.
### Expected behaviour
Executing `make install` should install all files successfully.
### Steps to reproduce
1. Run `make dist`
2. Extract `grip-tools.tar.gz` as folder
3. Change working directory to said folder (`cd ./grip-tools`) then run `make install`

Start with the Makefile and the packaging flow used by `make dist`; verify how `grip-tools-selfupdate.sh` is referenced in the generated archive. Run the listed `make dist` and `make install` steps, and consider the work complete when installation succeeds without the nonexistent-file failure.

Written by the indexing model from the issue text.