makeself: backport megastep/makeself#142

Currently, a self-extracting archive created by makeself will fail to
properly execute on NixOS because the boilerplate Bash code it uses to
clean up the temporary directory it extracted its contents into
assumes that the `rm` command is installed at `/bin/rm`, which is not
the case on NixOS.

This commit, a backport of a pull request I made to the upstream
repository at megastep/makeself#142, fixes the issue by causing the
boilerplate code to call `rm` without specifying an absolute path,
which allows the version of `rm` from one's current Nix environment to
be used instead.