nymea-plugins Fetch pip packages at build time

nymea-plugins pull request checklist:

[ ] Make sure the pull request's title is of format "Plugin name: Add support for xyz" or "New plugin: Plugin name"
[ ] Did you test the changes on hardware, if not (e.g. absence of required hardware), please mention a person to confirm it has been tested.
[ ] Did you update the plugin's README.md accordingly?
[ ] Did you update translations (cd builddir && make lupdate)?
[ ] If you added a new plugin, should it be added to nymea-plugins-all or nymea-plugins-maker?

May 10 '21 16:05 mzanetti

Refer to this link for build results (access rights to CI server needed): https://jenkins.nymea.io/job/nymea-plugins-pull-request-tester/1480/

May 10 '21 17:05 nymea-jenkins

Refer to this link for build results (access rights to CI server needed): https://jenkins.nymea.io/job/nymea-plugins-pull-request-tester/1479/

May 10 '21 17:05 nymea-jenkins

Refer to this link for build results (access rights to CI server needed): https://jenkins.nymea.io/job/nymea-plugins-pull-request-tester/1487/

May 12 '21 09:05 nymea-jenkins

Refer to this link for build results (access rights to CI server needed): https://jenkins.nymea.io/job/nymea-plugins-pull-request-tester/1493/