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.
ff3e7d2b4b