release arm64 binaries

[ccmtaylor]

Now that Go 1.16 is out, we can compile arm64 binaries for macOS.

I tested the resulting binary on my laptop (although I also built it on macOS/arm64). It seemed to work alright.

Fixes: #329

[ccmtaylor]

I think the failing validations are bogus. There is a nil check right above the line that the check is complaining about.

        if csrPEM == nil {
                log.Fatalln("ERROR: failed to read the CSR: unexpected content")
        }
        if csrPEM.Type != "CERTIFICATE REQUEST" &&    // this is line 220
                csrPEM.Type != "NEW CERTIFICATE REQUEST" {
                log.Fatalln("ERROR: failed to read the CSR: expected CERTIFICATE REQUEST, got " + csrPEM.Type)
        }

In any case, they are unrelated to this PR which doesn't touch any of the code, nor the CI test configuration.

[FiloSottile]

Ah, merged a duplicate (#433), my bad.