Expose parallel_build_count as a well-known variable to scriptlets

[Olivier Tilloy]

Use case: I had a custom plugin to build the 0AD snap, and I replaced it by scriptlets, which makes the packaging easier to maintain. Changes at https://bazaar.launchpad.net/~osomon/+junk/0ad-snap/revision/10.

One thing I was not able to achieve is invoke `make` from a scriptlet with the -jN flag. In custom plugins there is the 'parallel_build_count' property, it would be useful to expose it as a variable to scriptlets.

[Olivier Tilloy (osomon)]

Implemented as https://github.com/snapcore/snapcraft/pull/1154