Snap package should use snap configuration to set env variables

[bcardiff]

The following environment variables are used in shards and crystal for some workflows.

• SHARDS_OPTS
• SHARDS_OVERRIDE
• SHARDS_CACHE_PATH
• CRYSTAL_OPTS
• CRYSTAL_PATH
• CRYSTAL_LIBRARY_PATH
• CRYSTAL_CACHE_DIR

The snap package does not support them because of its sandboxing.

The right way to support them is to use snap configuration, and to use snapctl get inside the snap wrapper scripts for crystal and shards.

Ref: https://snapcraft.io/docs/adding-snap-configuration