Makefile: evaluate os/arch based on target name

[tstromberg]

Previously, the Makefile was setting a correct GOOS and GOARCH environment variables, but for flag values, it was passing in the native Makefile variables, set to the local OS and architecture that was executing the Makefile.

For example, running make out/skaffold-linux-amd64 on a macOS machine output:

GOOS="linux" GOARCH="amd64" \
	docker build \
		--build-arg GOOS=darwin \
		--build-arg GOARCH=amd64 \
		--build-arg TAGS="release" \
		--build-arg LDFLAGS=" -X 

In this PR, I change from setting an environment variable, to setting a local variable ("os" and "arch"), which is evaluated from the Makefile rule. This local variable is then passed in correctly as command-line arguments. The previous example now emits:

docker build \
		--build-arg GOOS=linux \
		--build-arg GOARCH=amd64 \
		--build-arg TAGS="osusergo netgo static_build release" \
		--build-arg LDFLAGS=" -X github.com/GoogleContainerTools/skaffold/pkg/skaffold/version.version=v1.10.0-4-gfafbfc163-dirty -X github.com/GoogleContainerTools/skaffold/pkg/skaffold/version.buildDate=2020-05-20T11:28:19Z -X github.com/GoogleContainerTools/skaffold/pkg/skaffold/version.gitCommit=fafbfc1636db2cfa49bcd0eb1ba0bd02fa921bb3 -X github.com/GoogleContainerTools/skaffold/pkg/skaffold/version.gitTreeState=dirty -s -w  -extldflags \"-static\"" \
		-f deploy/cross/Dockerfile \
		-t skaffold/cross \
		.

You'll notice that TAGS and LDFLAGS are now properly evaluated as well.

Fixes #4234

[tejal29]

its is frustrating, we cant use keys values pairs all though its supported on bash4.
https://stackoverflow.com/questions/6047648/bash4-associative-arrays-error-declare-a-invalid-option
It would have been much easier to read and less error prone.

this change looks good to me.

[codecov]

Codecov Report

Merging #4236 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
pkg/skaffold/util/cmd.go	47.61% <0.00%> (-4.77%)	⬇️
pkg/skaffold/deploy/kustomize.go	74.71% <0.00%> (-1.51%)	⬇️
pkg/skaffold/deploy/kubectl/cli.go	90.90% <0.00%> (-0.40%)	⬇️
...affold/kubernetes/portforward/kubectl_forwarder.go	68.29% <0.00%> (+2.43%)	⬆️