shellcheck: Fix warnings in the chutney script

Part of 30065.

chutney

@@ -3,11 +3,13 @@
 set -o errexit
 set -o nounset
 
-export PYTHONPATH="$(dirname "${0}")/lib:$(dirname "${0}")/scripts:${PYTHONPATH-}"
+PYTHONPATH="$(dirname "${0}")/lib:$(dirname "${0}")/scripts:${PYTHONPATH-}"
+export PYTHONPATH
 # make a last-ditch effort to set CHUTNEY_PATH based on this script's location
 if ! test "${CHUTNEY_PATH+y}"
 then
-    export CHUTNEY_PATH="`dirname $0`"
+    CHUTNEY_PATH=$(dirname "$0")
+    export CHUTNEY_PATH
 fi
 
 binaries="python python3 python2"