dashboard: set SkipSnapshot on freebsd-arm and dragonfly-amd64
dashboard: set SkipSnapshot on freebsd-arm and dragonfly-amd64

No point snapshotting for non-sharded builders.

And snapshotting is too slow and hits the timeout on freebsd-arm.

Updates golang/go#21191

Change-Id: I225aa6af493b92aa45589491c69853e13d513dd1
Reviewed-on: https://go-review.googlesource.com/53493
Reviewed-by: Yuval Pavel Zholkover <170f3b94ed768c88b460d7f0c362c1d8ffaf0cec@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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