Incorrect calling of another plugin/command

[purplecabbage]

https://github.com/adobe/aio-cli-plugin-app/blame/251f7bb62424d52d9eef307a161d28b3ddcc9d9f/src/commands/app/pack.js#L139

I missed this when it was committed, but this is not how we should call other commands in the app-plugin.

The proper way to do this is here:

aio-cli-plugin-app/src/commands/app/run.js

Lines 154 to 157 in 251f7bb

	const CertCmd = this.config.findCommand('certificate:generate')
	if (CertCmd) {
	const Instance = await CertCmd.load()
	await Instance.run([`--keyout=${PRIVATE_KEY_PATH}`, `--out=${PUB_CERT_PATH}`, '-n=DeveloperSelfSigned.cert'])

[aiojbot]

JIRA issue created: https://jira.corp.adobe.com/browse/ACNA-2555