env: add gfortran for linux builders
env: add gfortran for linux builders

Fix golang/go#14488

Change-Id: I29f77eafcf34963cc05fc956dc6265eaae9718d2
Reviewed-on: https://go-review.googlesource.com/30710
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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