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.

pkgs/build-support/setup-hooks/patch-shebangs.sh

@@ -46,7 +46,8 @@ patchShebangs() {
             args="$arg0 $args"
         fi
 
-        newInterpreterLine="$newPath $args"
+        # Strip trailing whitespace introduced when no arguments are present
+        newInterpreterLine=$(echo "$newPath $args" | sed 's/[[:space:]]*$//')
 
         if [ -n "$oldPath" -a "${oldPath:0:${#NIX_STORE}}" != "$NIX_STORE" ]; then
             if [ -n "$newPath" -a "$newPath" != "$oldPath" ]; then