default.nix: remove bundler from runtime environment - it is not needed
default.nix: remove bundler from runtime environment - it is not needed

It also causes version mismatch problems with rubygems' bundled... bundler.
`bundle exec` is not needed in the execution commands.

alphagov/digitalmarketplace-functional-tests,alphagov/digitalmarketplace-functional-tests,alphagov/digitalmarketplace-functional-tests