Prepend 0 to auto-computed WEB_CONCURRENCY values

When customers run Node together with PHP or Python, then Node's .profile.d/ script will set a WEB_CONCURRENCY variable depending on dyno size. The Python .profile.d script (due to alphabetical order) and the PHP "boot scripts" then see a WEB_CONCURRENCY value and use it, as they cannot know that the variable was set not by a customer using heroku config:set, but by another buildpack's dyno .profile.d/ script.

This change prepends a leading zero to the WEB_CONCURRENCY variable set by the Node buildpack, which the Python and PHP buildpacks can detect and choose to ignore, but should not affect customers using the variable.

JS has some weirdness with leading zeros being interpreted as octal, but that doesn't appear to affect us here.