h2t/research_projects/oml/mycroft/mycroft-core: Export env variable for web cache config

Closes #223 (closed)

Export MYCROFT_WEB_CACHE to override default path (/var/tmp/mycroft_web_cache.json), which causes permission errors on machines with multiple accounts (e.g. robot).

The new location would be $ARMARX_WORKSPACE/.../mycroft-core/mycroft_web_cache.json. This is similar to mycroft.conf, which I moved from the home directory to the Git repository to put it under version control.

Should fix https://git.h2t.iar.kit.edu/orga/software-organization/-/issues/248