ci: don't compile gotestsum

[landism]

Problem:

the macos CI build keeps failing with errors like this:

#!/bin/bash -eo pipefail
go get -u gotest.tools/gotestsum
        go get: warning: modules disabled by GO111MODULE=auto in GOPATH/src;
	ignoring go.mod;
	see 'go help modules'
# cd .; git clone https://go.googlesource.com/sys /Users/distiller/go/src/golang.org/x/sys
Cloning into '/Users/distiller/go/src/golang.org/x/sys'...
fatal: remote error: Internal Server Error
package golang.org/x/sys/unix: exit status 128
Exited with code 1

Solution:
download gotestsum instead of go getting it

(also this step was taking 30-40 seconds, so we might as well not do that?)