Fix script matching in exercise.sh

This prevents false matches in exercise skip test

Change-Id: I5656a20bcf11b2ccaf55e280655d0600124adedc
diff --git a/exercise.sh b/exercise.sh
index a0349ce..5b3c56e 100755
--- a/exercise.sh
+++ b/exercise.sh
@@ -28,7 +28,7 @@
 
 # Loop over each possible script (by basename)
 for script in $basenames; do
-    if [[ "$SKIP_EXERCISES" =~ $script ]] ; then
+    if [[ ,$SKIP_EXERCISES, =~ ,$script, ]] ; then
         skips="$skips $script"
     else
         echo "====================================================================="