switch to go build -cover for integration tests

[cyphar]

With Go 1.20, we can now avoid using the whole go test -c -cover setup by simply building the regular binary with coverage profiling enabled, which should make all of our testing logic much simpler.

I suspect codecov doesn't support the new coverage format so we will need to use go tool covdata textfmt to convert back to the format it understands (though thankfully the new binary format can auto-merge everything without needing the awk script I wrote for that purpose).