PR: Fix installer triggers

[mrclary]

Opening a PR and labeling it at the same time results in a race condition between the opened and labeled pull_request event triggers which can lead to cancelled installer workflows.

To fix this, we remove the labeled event trigger for pull_requests.
Since release candidates were the primary motivation for these triggers, but this must rely on the workflow_dispatch trigger anyway, the workflow_dispatch is now the recommended way to trigger workflows outside of pull_request and release events.

Additionally, inputs for the workflow_dispatch were added to the conda-based installer workflow for enhanced flexibility.

• pull_request.labeled trigger events are removed for installer workflows
• push.tag trigger events are removed for installer workflows
• workflow_dispatch trigger is added to unit test workflows. These can now be triggered to run on arbitrary branches, independent of pull_requests.
• inputs are added to the workflow_dispatch in conda-based installer workflow
  • "Build as release candidate": build the installers using released versions of qtconsole, python-lsp-server, and spyder-kernels; codesign and notarize the resulting installers (macOS only). If deselected, qtconsole, python-lsp-server, and spyder-kernels are built from the upstream remote and commit specified in external-deps
  • "Build macOS installer": build the macOS installer
  • "Build Linux installer": build the Linux installer
  • "Build Windows installer": build the Windows installer

[ccordoba12]

Looks good to me, thanks @mrclary!