feat(server): Use hash of Nixery source as version

Uses a hash of Nixery's sources as the version displayed when Nixery
launches or logs an error. This makes it possible to distinguish
between errors logged from different versions.

The source hashes should be reproducible between different checkouts
of the same source tree.

tools/nixery/server/logs.go

@@ -27,7 +27,6 @@ type reportLocation struct {
 
 var nixeryContext = serviceContext{
 	Service: "nixery",
-	Version: "TODO(tazjin)", // angry?
 }
 
 // isError determines whether an entry should be logged as an error
@@ -63,6 +62,7 @@ func (f stackdriverFormatter) Format(e *log.Entry) ([]byte, error) {
 }
 
 func init() {
+	nixeryContext.Version = version
 	log.SetReportCaller(true)
 	log.SetFormatter(stackdriverFormatter{})
 }