databases/influxdb: Update to 1.8.0

• Convert to USES=go:modules to simplify port maintenance
• Add post-extract to pet go:modules instructions
• Improve whitespace in Makefile

Changes this release:

https://docs.influxdata.com/influxdb/v1.8/about_the_project/releasenotes-changelog/

PR: 246844
Submitted by: Daniel Tihanyi <daniel.tihanyi@tetragir.com>