jsonnet-bundler binary env var

[sh0rez]

Currently we hard-code to use the binary called jb on the $PATH in init.go:

tanka/cmd/tk/init.go

Lines 74 to 76 in 6941b87

	if _, err := exec.LookPath("jb"); err != nil {
	return errors.New("jsonnet-bundler not found in $PATH. Follow https://tanka.dev/install#jsonnet-bundler for installation instructions")
	}

To be more resilient to differently set up environments, we should allow to set an TANKA_JB_PATH, similar to the existing TANKA_KUBECTL_PATH: https://tanka.dev/env-vars#tanka_kubectl_path

---

Steps to implement:

1. Use os.Getenv("TANKA_JB_PATH") to read the env var, but default to "jb" if unset, as already done for kubectl:
   

   tanka/pkg/kubernetes/client/exec.go

   Lines 14 to 17 in 6941b87

   	binary := "kubectl"
   	if env := os.Getenv("TANKA_KUBECTL_PATH"); env != "" {
   	binary = env
   	}

2. Put the result of that into the exec.LookPath call, keep the error checking:
   

   tanka/cmd/tk/init.go

   Lines 74 to 76 in 6941b87

   	if _, err := exec.LookPath("jb"); err != nil {
   	return errors.New("jsonnet-bundler not found in $PATH. Follow https://tanka.dev/install#jsonnet-bundler for installation instructions")
   	}

3. Update the docs to include the new env var in docs/docs/env-vars.md.