rhcos-toolbox: use interactive flag for exec

After the initial execution of toolbox, subsequent executions
would fail because the interactive flag is missing.  The user is
left with a shell prompt but cannot type anything.
diff --git a/rhcos-toolbox b/rhcos-toolbox
index 2052032..73b11ea 100755
--- a/rhcos-toolbox
+++ b/rhcos-toolbox
@@ -118,6 +118,7 @@
             --env LANG="$LANG" \
             --env TERM="$TERM" \
             --tty \
+            --interactive \
             "$TOOLBOX_NAME" \
             "$@"
 }