Use the ramsey/composer-install action to install dependencies

[nicwortel]

While migrating doctrine/data-fixtures from Travis CI to GitHub Actions (doctrine/data-fixtures#353), looking for an elegant way to add a job with the lowest dependency versions I came across the composer-install action made by @ramsey.

This action makes it easy to choose between installing locked versions (composer install), highest versions (composer update), and lowest versions (composer update --prefer-lowest). The --no-interaction, --no-progress, and --ansi flags are automatically added to the command. It also handles caching of dependencies to improve build times.

In other words, something like this:

      - name: "Cache dependencies installed with composer"
        uses: "actions/cache@v2"
        with:
          path: "~/.composer/cache"
          key: "php-${{ matrix.php-version }}-composer-locked-${{ hashFiles('composer.lock') }}"
          restore-keys: "php-${{ matrix.php-version }}-composer-locked-"

      - name: "Install dependencies with composer"
        run: "composer update --no-interaction --no-progress --no-suggest"
        if: "${{ matrix.dependencies == 'highest' }}"

      - name: "Install lowest possible dependencies with composer"
        run: "composer update --no-interaction --no-progress --no-suggest --prefer-dist --prefer-lowest"
        if: "${{ matrix.dependencies == 'lowest' }}"

can be rewritten to this:

      - name: "Install dependencies with composer"
        uses: "ramsey/composer-install@v1"
        with:
          dependency-versions: "${{ matrix.dependencies }}"
          composer-options: "--prefer-dist --no-suggest"

I believe this greatly improves the readability and maintainability of GitHub Actions workflows in the Doctrine organization.

As suggested by @greg0ire I'm opening this PR to discuss the usage of this GitHub Action on an organization level.

[SenseException]

Tests of the action seem to cover the replaced steps.

[greg0ire]

Thanks @nicwortel, I think you can go ahead!