dashboard: add OpenBSD 6.8 builders
dashboard: add OpenBSD 6.8 builders

This change builds on top of CL 268297, using the updated env script
to build OpenBSD 6.8 images, and adding new builders that use them.

This has been tested with cmd/debugnewvm on a recent Go tip commit,
as well as Go 1.15.4 and 1.14.11 commits. It passed, although there
were also some failures seen. Once the builder is added, we'll know
the test results for a wider set of commits, and can decide how to
proceed from there.

Minimize the configuration to a bare minimum. Specifically, remove
MinCPUPlatform since setting it "Intel Skylake" was found not to
have a noticeable effect on the speed of the build compared to the
zero value:

	# With MinCPUPlatform = "Intel Skylake":
	$ time debugnewvm -run-build=openbsd-amd64-68 -rev=go1.15.4
	[...]
	real	17m8.308s

	# With a zero MinCPUPlatform value:
	$ time debugnewvm -run-build=openbsd-amd64-68 -rev=go1.15.4
	[...]
	real	16m28.518s

For golang/go#35712.
Updates golang/go#42426.

Change-Id: I517a85738a034a3a4074488ccecd4408a91a4a07
Reviewed-on: https://go-review.googlesource.com/c/build/+/269577
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

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