add `project-update` flag

[joaofnds]

Add project-update flag, defaulting to 'false', to install deps before running args

Adding this flag would enable this action:

    steps:
      - name: Checkout
        uses: actions/checkout@v2

      - name: dagger do test
        uses: dagger/dagger-for-github@v2
        with:
          install-only: true

      - run: dagger project update
      - run: dagger do test

to be like this:

    steps:
      - name: Checkout
        uses: actions/checkout@v2

      - name: dagger do test
        uses: dagger/dagger-for-github@v2
        with:
          project-update: true
          args: do test

You can see this version running here: https://github.com/joaofnds/ds/commit/76d0c94d675e4503db5f8ee8e3d05280a82032ac https://github.com/joaofnds/ds/runs/5956403863?check_suite_focus=true#step:3:14

closes: #43

[marcosnils]

:wave: dagger project update also has an optional [package] that can be specified through the CLI to optionally pull dependencies from a third-party repo. I don't see that it's being taken into consideration here. My suggestions is to change the project-update flag so it can optionally receive a value to fetch deps from the non-default location.