fix: pass json flag to plugins install

[shetzel]

What does this PR do?

If a JIT plugin command is run with --json and that plugin is not yet installed (such as in a CI system) the output from the JIT install (which is not JSON) would mix with the JSON to cause problems. This fix passes along the --json flag to the call to plugins:install so the output is still valid JSON.

E.g., sf package version create ... --json would cause output like:

Successfully installed @salesforce/plugin-packaging v1.27.0
  {
    "status": 0,
    "result": {
      "Id": "<PACKAGE_ID>",
      ...
    },
    "warnings": []
  }

With the fix, the output contains only JSON.

What issues does this PR fix or reference?

@W-14457843@
forcedotcom/cli#2562

[mshanemc]

QA:

(unintentionally): wrote a command that errored out (wrong alias).
✅ good json error to stdout, everything else to stderr.

correct command
✅ good json output to stdout

📓 warnings are going to stderr. sfCommand usually gets these into the warnings of its json, but here we're losing them. Acceptable given the circumstances, worth considering in the future.