Makefile: enable CGO for make test in dev shell

[sfllaw]

Fixes #3401

• have read the CONTRIBUTING.md file
• raised a GitHub issue or discussed it on the projects chat beforehand
• added unit tests
• added integration tests
• updated documentation if needed
• updated CHANGELOG.md

Testing

sfllaw@h2co3:~/Projects/headscale/make-test-with-cgo-enabled$ nix develop
sfllaw@h2co3:~/Projects/headscale/make-test-with-cgo-enabled$ echo $CGO_ENABLED
0
sfllaw@h2co3:~/Projects/headscale/make-test-with-cgo-enabled$ make test
Running Go tests...
CGO_ENABLED=1 go test -race ./...
?   	github.com/juanfont/headscale/cmd/dev	[no test files]
?   	github.com/juanfont/headscale/cmd/gen-openapi	[no test files]
ok  	github.com/juanfont/headscale/cmd/headscale	1.053s