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
```
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