www/grafana7: add dependency on ca_root_nss

Logs contains all kind of certificate errors if ca_root_nss is not
installed.

t=2021-04-20T13:55:49+0200 lvl=dbug msg="Failed to get plugins repo from grafana.com, Get \"https://grafana.com/api/plugins/versioncheck?slugIn=grafana-piechart-panel,grafana-clock-panel,grafana-worldmap-panel,savantly-heatmap-panel&grafanaVersion=7.5.1\": x509: certificate signed by unknown authority"

Effectively it is not possible to install dashboards/etc. from the
grafana site without this dependency install. Installing ca_root_nss
and restarting grafana resolved the problem immediately.

PR: 255255
Approved by: maintainer timeout (2+ weeks)

(cherry picked from commit cf3eb1d087d2245f8961724e4d7d54b5f8fdd8ac)