Bump erlef/setup-beam from 1.7 to 1.8.1

Bumps [erlef/setup-beam](https://github.com/erlef/setup-beam) from 1.7 to 1.8.1.
- [Release notes](https://github.com/erlef/setup-beam/releases)
- [Commits](https://github.com/erlef/setup-beam/compare/v1.7...v1.8.1)

---
updated-dependencies:
- dependency-name: erlef/setup-beam
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/elixir.yml

@@ -35,7 +35,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2.3.4
-      - uses: erlef/setup-beam@v1.7
+      - uses: erlef/setup-beam@v1.8.1
         with:
           otp-version: ${{ matrix.pair.otp }}
           elixir-version: ${{ matrix.pair.elixir }}