Support running tests on arm processors

[wdbaruni]

Builds are failing on Mac with arm processors with the following errors:

testground run single --plan example --testcase output  --builder exec:go --runner local:exec  --instances 3 --wait
Sep  7 20:35:26.172736  ERROR   go build failed: # golang.org/x/sys/unix
/Users/walid/.go/pkg/mod/golang.org/x/sys@v0.0.0-20200625212154-ddb9806d33ae/unix/syscall_darwin.1_13.go:29:3: //go:linkname must refer to declared function or variable
/Users/walid/.go/pkg/mod/golang.org/x/sys@v0.0.0-20200625212154-ddb9806d33ae/unix/zsyscall_darwin_arm64.1_13.go:27:3: //go:linkname must refer to declared function or variable

Running the same example test passes by overriding sdk-go dependency to use the version in this PR

vi testground/plans/example/go.mod
replace github.com/testground/sdk-go => /Users/walid/ProtocolLabs/workspace/testground-sdk-go

> testground run single --plan example --testcase output  --builder exec:go --runner local:exec  --instances 3 --wait
// passes