Automatic merge from submit-queue (batch tested with PRs 45247, 45810, 45034, 45898, 45899)

kubectl plugin: Expand environment variables in the command of plugin.yaml

**What this PR does / why we need it**:

This commit improves kubectl plugins to expand environment variables in the command of plugin.yaml.
I'd like to use environment variables in the command of plugin.yaml as follows:
```yaml
name: hello
shortDesc: "The hello plugin"
longDesc: >
  The hello plugin is a new
  plugin used by test-cmd
  to test multiple plugin locations.
command: $HOME/path/to/plugins/hello.sh
```

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```