Consolidate to single style for specifying environment-style parameters (env, buildArgs)

[briandealwis]

We have three styles of specifying environment-style variables in the skaffold.yaml. It's inconsistent and confusing for users (#5841).

Map-style

Seen in Docker and Kaniko's buildArgs:

        // BuildArgs are arguments passed to the docker build.
        // For example: `{"key1": "value1", "key2": "value2"}`.
        BuildArgs map[string]*string `yaml:"buildArgs,omitempty"`

The use of a pointer here allows using YAML's null to unset a value

  docker:
     buildArgs:
       name: value
       other: null

Array-style

Seen in Buildpacks's env:

        // Env are environment variables, in the `key=value` form,  passed to the build.
        // Values can use the go template syntax.
        // For example: `["key1=value1", "key2=value2", "key3={{.ENV_VARIABLE}}"]`.
        Env []string `yaml:"env,omitempty"`

Key-Value pairs (k8s.io/api/core/v1.EnvVar)

Seen in Kaniko's env:

        // Env are environment variables passed to the kaniko pod.
        // It also accepts environment variables via the go template syntax.
        // For example: `[{"name": "key1", "value": "value1"}, {"name": "key2", "value": "value2"}, {"name": "key3", "value": "'{{.ENV_VARIABLE}}'"}]`.
        Env []v1.EnvVar `yaml:"env,omitempty"`

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We have other fields that are in a similar state:

• Kaniko label is a map[string]*string
• Cluster's annotations is a map[string]string