evaluate: Leave GUILE_LOAD_PATH unchanged.

Previously we'd systematically add the first argument to the search
path.  When that first argument was the empty string (as with the
'guix-modular' jobset), we'd thus add the current directory to the
search path, even if the intent was to leave the load path unchanged.

Furthermore, a3a7c09b changed load-path
handling to be explicit in Scheme.

* bin/evaluate.in: Remove GUILE_LOAD_PATH assignment.

bin/evaluate.in

 #!/bin/sh
 # -*- scheme -*-
 # @configure_input@
-GUILE_LOAD_PATH="$1${GUILE_LOAD_PATH:+:}$GUILE_LOAD_PATH"
-export GUILE_LOAD_PATH
 exec ${GUILE:-@GUILE@} --no-auto-compile -e main -s "$0" "$@"
 !#
 ;;;; evaluate -- convert a specification to a job list