Cache coverage reports against master

[aconrad]

WIP: currently using Github Action cache to store coverage artifacts of master branch. But artifacts are only accessible for a week.

A repository can have up to 10GB of caches. Once the 10GB limit is reached, older caches will be evicted based on when the cache was last accessed. Caches that are not accessed within the last week will also be evicted.

Source.

Given how infrequent the development against master is, the coverage generated by the master branch is unlikely to be available the next time a new PR is opened and coverage diff won't be able to work.

[goatwu1993]

I had setup a similar workflow, PR comment included, using this awesome library(if anyone is interested...)