Add Go 1.16 to prerequisites (contributing doc)

[stefanprodan]

Changes to contributing doc:

• add image automation repos to GOTK list
• add prerequisites for running the tests locally

[hiddeco]

This has never worked on my system, as /usr/local/bin is protected, and I do not run my make commands with sudo:

$ make install-dev
CGO_ENABLED=0 go build -o /usr/local/bin ./cmd/flux
go build github.com/fluxcd/flux2/cmd/flux: copying /tmp/go-build3338692359/b001/exe/a.out: open /usr/local/bin/flux: permission denied
make: *** [Makefile:35: install-dev] Error 1

If you change it to something like this, it will prompt for a password, while (hopefully) not creating any problems on MacOS:

Suggested change

	install-dev: $(EMBEDDED_MANIFESTS_TARGET)
	CGO_ENABLED=0 go build -o /usr/local/bin ./cmd/flux
	install-dev: $(EMBEDDED_MANIFESTS_TARGET)
	CGO_ENABLED=0 go build -o /tmp/flux-dev ./cmd/flux
	sudo install -Dm755 /tmp/flux-dev /usr/local/bin/flux

This results in the following output, which should make clear why it asks for a password:

$ make install-dev
CGO_ENABLED=0 go build -o /tmp/flux-dev ./cmd/flux
sudo install -Dm755 /tmp/flux-dev /usr/local/bin/flux
[sudo] password for hidde:

[stefanprodan]

Seems that we are in a deadlock here, you don't want sudo make and I don't want make sudo.

[hiddeco]

sudo make would make Go run as root, which is not something you want as all Go Modules are cached for the normal user.

My suggestion makes this target work on any system, and the issue you have with it for MacOS (or more specifically, Homebrew) can be overcome by running brew with the --overwrite flag. Given all this, I see much stronger arguments in favor of my suggestion, than against.

[stefanprodan]

I've undone the changes to Makefile