Prior to this commit, `parseEnvs` accept two parameters:

  1. env []string
  2. envs map[string]string

`parseEnvs` then do a `nil` check for `env`. However, we never pass a
`nil` `env` to `parseEnvs` in `newRunCommand`.

This commit simplify the `parseEnvs` function to accept just one
`env []string` parameter and return the result as `map[string]string`
instead.

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>