fix: prevent postrun hook from running on uninstalled plugins

[mdonnalley]

Fixes #804

@W-14226831@

[cristiand391]

plugins uninstall only accepts 1 arg but I'm guessing argv could also include --verbose. this.plugins is a map and delete doesn't throw if an element doesn't exist so this seems safe.

[mdonnalley]

Yes but I think plugins uninstall is supposed to take multiple arguments: https://github.com/oclif/plugin-plugins/blob/main/src/commands/plugins/uninstall.ts#L16

I'm assuming at some point variableArgs = true was changed to strict = false but the command was never updated. Iterating over the args now future proofs us from that being fixed eventually

[cristiand391]

QA notes:

repro'd with hardis plugin on latest sf RC.
test setup:
sf repo with this branch of core linked into it, all deduped to ensure only this branch is used and a linked plugin with a postrun hook that logs to stdout:

import {Hook} from '@oclif/core'

const hook: Hook<'postrun'> = async function (opts) {
  process.stdout.write(`argv: ${opts.argv}`)
}

export default hook

✅ ./bin/run plugins uninstall sfdx-hardis uninstalls the plugin and doesn't try to fire postrun hook from hardis plugin, runs postrun hook from linked plugin
✅ ./bin/run plugins uninstall sfdx-hardis --verbose same as previous test with a bunch of output from yarn, output from test hook:

...
Uninstalling sfdx-hardis... ⡿
success Uninstalled packages.
Uninstalling sfdx-hardis... done
argv: sfdx-hardis,--verbose%

✅ unlink plugin with postrun hook, no output to stdout:

➜  sf-cli git:(main) ./bin/run plugins uninstall @cristiand391/sf-plugin-api
Uninstalling @cristiand391/sf-plugin-api... done

note: found another bug with the hook generator, see: oclif/oclif#1196