Don't print out version when executing the proxy command

Because the proxy reads from stdin and write to stdout on one side of the connection, printing anything to stdout breaks the whole thing. The relevant error message is.

    executing step request: running job request: rpc error: code = Unavailable desc = connection error: desc = "error reading server preface: http2: frame too large"

To fix the above I had to move printing of the version to the root command, where I can detect what subcommand is being run. To to the latter I had to make some changes to how ldflags are handled.