Fix rockspec and version injection

[Totktonada]

This patchset should actually finish the implementatoin of the graphql.VERSION string generation.

Follows up #29.

[Totktonada]

There are several alternatives:

• Use build.type = 'cmake' in the rockspec and call git describe from cmake by @no1seman. See #50.
• Use build.type = 'make' in the rockspec and call git describe from make (my PoC). See 332b1ab0060711263bd7599d2aeaf75210d2b054.
• Continue to generate the version file from CI. This patchset.

The unobvious thing about first two options is that they only work while tarantoolctl rocks make/pack is invoked from inside the git repository (see https://github.com/tarantool/graphql/pull/50#issuecomment-1086989280). Burrying it down to the tarantoolctl rocks make/pack level just hides what is actually going on.

So I prefer the simple and explicit variant and propose it here.