include go build output in build error

[imjasonh]

This doesn't change much for users of ko as a CLI, the output still contains relevant error output:

$ go run ./ build ./test/                                 
2023/09/07 09:18:11 Using base cgr.dev/chainguard/static:latest@sha256:a432665213f109d5e48111316030eecc5191654cf02a5b66ac6c5d6b310a5511 for github.com/google/ko/test
2023/09/07 09:18:11 Building github.com/google/ko/test for linux/amd64
Error: failed to publish images: error building "ko://github.com/google/ko/test": build: go build: exit status 1: # github.com/google/ko/test
test/main.go:80:1: syntax error: non-declaration statement outside function body

Previously, this was:

ko build ./test/
2023/09/07 09:19:21 Using base cgr.dev/chainguard/static:latest@sha256:a432665213f109d5e48111316030eecc5191654cf02a5b66ac6c5d6b310a5511 for github.com/google/ko/test
2023/09/07 09:19:22 Building github.com/google/ko/test for linux/amd64
2023/09/07 09:19:22 Unexpected error running "go build": exit status 1
# github.com/google/ko/test
test/main.go:79:1: syntax error: non-declaration statement outside function body
Error: failed to publish images: error building "ko://github.com/google/ko/test": build: go build: exit status 1
exit status 1

For users of ko as a library, mainly tf-ko, which swallows log output, this will be very helpful in surfacing the go build error.