godoc/static: tweak regular expression to match source builds

(This time for sure!)

Change-Id: Ic9b7dbfb527af9aa029365f7a91bd3b949985431
Reviewed-on: https://go-review.googlesource.com/13220
Reviewed-by: Chris Broadfoot <cbro@google.com>
Reviewed-by: Andrew Gerrand <adg@golang.org>