ci: follow-up improvements for gc64 builds

This change contains the following improvements:

1. Prettify view in the job list.

   Before:
     - centos_7 / centos_7 (GC64=OFF)
     - centos_7 / centos_7 (GC64=ON)

   After:
     - centos_7 / centos_7
     - centos_7 / centos_7 (gc64)

2. Change possible values for the ${GC64} env variable to
   `true/false` instead of `ON/OFF` which is more traditional.

3. Use `-DLUAJIT_ENABLE_GC64=ON` flag in the `debian/rules` and
   `rpm/tarantool.spec` files directly instead of providing it
   in the workflow files.

Follows-up tarantool/tarantool-qa#159
Follows-up tarantool/tarantool-qa#161

NO_DOC=ci
NO_TEST=ci
NO_CHANGELOG=ci