Free up disk space for Github Actions runner

Free up disk space for Github Actions runner since the `test` stage
on the `Build + Test` workflow started to face disk space issues
after the upgrade to libp2p 0.42.2.
This removes some tools as stated in [this
comment](actions/runner-images#2840 (comment)).

.github/workflows/build+test.yml

@@ -33,6 +33,8 @@ jobs:
     runs-on: ubuntu-20.04
 
     steps:
+    - name: "Free up disk space"
+      run: sudo rm -rf /usr/share/dotnet && sudo rm -rf /opt/ghc && sudo rm -rf "/usr/local/share/boost" && sudo rm -rf "$AGENT_TOOLSDIRECTORY"
     - uses: actions/checkout@v2
     - uses: actions/cache@v2
       with: