cmd/buildlet: fix a forgotten rename from buildlet type to host type

I missed this in the great rename between patchsets 6 and 7:

https://go-review.googlesource.com/#/c/29551/6..7

Change-Id: Id34d36805519a7baa2ab1ea96f616ee1705994b0
Reviewed-on: https://go-review.googlesource.com/29671
Reviewed-by: Matthew Dempsky <[email protected]>

Author: bradfitz

cmd/buildlet/buildlet.go

@@ -1242,9 +1242,9 @@ func makeBSDFilesystemFast() {
 		log.Printf("Not on GCE; not remounting root filesystem.")
 		return
 	}
-	btype, err := metadata.InstanceAttributeValue("buildlet-type")
+	btype, err := metadata.InstanceAttributeValue("buildlet-host-type")
 	if _, ok := err.(metadata.NotDefinedError); ok && len(btype) == 0 {
-		log.Printf("Not remounting root filesystem due to missing buildlet-type metadata.")
+		log.Printf("Not remounting root filesystem due to missing buildlet-host-type metadata.")
 		return
 	}
 	if err != nil {