src extensions publish does not work correctly on Windows

[felixfbecker]

https://github.com/sourcegraph/src-cli/blob/4e548d9ca9a9a4316bf4692773f44bd19f5386dc/cmd/src/extensions_publish.go#L174

The expected behaviour here would be what npm does, which is using cmd on Windows. This decision is automatically made by Node child_process.exec().

https://github.com/sourcegraph/src-cli/blob/4e548d9ca9a9a4316bf4692773f44bd19f5386dc/cmd/src/extensions_publish.go#L175

Windows uses ; to separate PATH, not :.