The installed toolchain should become the default

[aminya]

Do the checklist before filing an issue:

• Is this related to the actions-rs Actions?
  If you think it's a problem related to Github Actions in general, use GitHub Community forum instead: https://github.community
• You've read the Contributing section about feature requests: https://github.com/actions-rs/.github/blob/master/CONTRIBUTING.md#feature-requests
• Is this something you can debug and fix? Send a pull request! Bug fixes and documentation fixes are welcome.

Motivation

The current behaviour for not setting the installed toolchain as default is very unintuitive. When I use action-rs to install a toolchain, I am expecting it to be the toolchain that I will use.

Workflow example

      - name: Setup rust
        uses: actions-rs/toolchain@v1
        with:
          toolchain: ${{ matrix.rust }}
          default: true       # this should be true by default!

Additional context

This has caused me a lot of trouble when I wanted to use a nightly feature, but no matter what I did I could not find the reason. Until I realized that action-rs does not set the installed nightly toolchain as default, and the rustc installed on the GitHub images were being used.

Then this is the moment that I realized that for such a long time, even for the projects that use a stable toolchain, action-rs has been just downloading a toolchain consuming the minutes I paid for in the CI without having any impact on the build.