cmd/buildlet: fix a forgotten rename from buildlet type to host type
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 <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

golang/build,golang/build,golang/build,golang/build