set GOARM for older arm variants

[mattmoor]

Prior to aarch64 to build for arm variants you needed to set GOARCH=arm GOARM=v6 or whatnot.

I don't believe we currently set this, although I believe it appears in the platform info in the manifest list.

cc @jonjohnsonjr

I noticed this building a ko image on golang, which has many many variants.

[jonjohnsonjr]

From https://golang.org/cmd/go/:

GOARM

For GOARCH=arm, the ARM architecture for which to compile.
	Valid values are 5, 6, 7.

$ crane manifest golang | jq .manifests[].platform.variant
2020/11/04 10:24:45 No matching credentials were found, falling back on anonymous
null
"v5"
"v7"
"v8"
null
null
null
null
null
null

Hate to see that v8. I guess we are going to hardcode a bunch of arm specific things.

Open issue that we can TODO: golang/go#29373