dashboard: fix bootstrap path for s390x builder
dashboard: fix bootstrap path for s390x builder

Change-Id: I8d28dfdb18da11e64cd94c7d205990f64793fcc6
Reviewed-on: https://go-review.googlesource.com/20168
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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