Eschew use of `sed`

At least on the Mac, the `sed` command silently fails, not replacing
`LOCALUSERNAME`.
There, a correct invocation is something like:
```
sh "sed -i '' -e 's/LOCALUSERNAME/#{ENV['USER']}/' #{dst_path}"
```

Rather than comprehensively account for differences in `sed` on various
platforms, use Ruby's File class to manipulate the files.