patchShebangs: strip trailing whitespace

Prior to this commit, trailing whitespace would be introduced when
modifying '#!' lines with no arguments.  For example (whitespace added):

    /nix/store/.../foo: interpreter directive changed
        from "/bin/bash"
          to "/nix/store/...-bash-4.3-p42/bin/bash  "

    /nix/store/.../bar: interpreter directive changed
        from "/bin/baz wef"
          to "/nix/store/...-baz wef "

We add a sed command to strip trailing whitespace, so the above commands
would drop the two spaces after "bash", or the one space after "baz wef".

abbradar: fixed commit title

Closes #16785.