upload dashboards via pipe due to argument list too long

Seeing the CI pipeline on master failing with (sample):

$ dashboards/upload.sh
Installed https://dashboards.gitlab.net/d/web-pages-main/web-pages-overview
grafana-tools.lib.sh: line 10: /usr/bin/curl: Argument list too long
API call to https://dashboards.gitlab.net/api/dashboards/db failed: : exit code 126

This patch uses curl -d @- to read from stdin, bypassing the massive blob being passed as an argument.