SBOM generation fails on Windows

https://github.com/google/ko/runs/4508967288?check_suite_focus=true#step:4:91

```
++ go run ./ publish ./ --platform=windows/amd64 --preserve-import-paths
2021/12/13 16:28:50 Using base mcr.microsoft.com/windows/nanoserver:1809 for github.com/google/ko
2021/12/13 16:28:50 Building github.com/google/ko for windows/amd64
C:\Users\RUNNER~1\AppData\Local\Temp\ko2792353245\out: not executable file
2021/12/13 16:29:33 Loading ko.local/github.com/google/ko:f16f6b389d4d4ccfc1db0524e5f8ec6b315fc6a4e080f0ffe17f5074aa4a6d2a
2021/12/13 16:29:34 Loaded ko.local/github.com/google/ko:f16f6b389d4d4ccfc1db0524e5f8ec6b315fc6a4e080f0ffe17f5074aa4a6d2a
2021/12/13 16:29:34 Adding tag latest
2021/12/13 16:29:34 Added tag latest
```

The binary built by `go build` on Windows doesn't seem to be executable when we run `go version -m` on it, which causes SBOM generation to fail. Interestingly, this doesn't seem to block pushing the rest of the image.

cc @mattmoor