Add github actions workflow for testing PRs (#959) #4

Job	Run time
Build Prometheus for common architectures	0s
Go tests	1m 28s
Build Prometheus for all architectures (0)	1m 20s
Build Prometheus for all architectures (1)	1m 25s
Build Prometheus for all architectures (2)	1m 16s
Build Prometheus for all architectures (3)	1m 23s
Build Prometheus for all architectures (4)	1m 23s
Build Prometheus for all architectures (5)	1m 25s
Build Prometheus for all architectures (6)	1m 24s
Build Prometheus for all architectures (7)	1m 22s
Build Prometheus for all architectures (8)	1m 22s
Build Prometheus for all architectures (9)	1m 17s
Build Prometheus for all architectures (10)	1m 18s
Build Prometheus for all architectures (11)	1m 17s
Publish main branch artifacts	39s
Publish release artefacts	0s
	18m 19s

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