Pin staticcheck version to v0.2.2 (#63)

The CI pipeline has been broken in this project for some time because
the latest version of staticcheck is not compatible with every version
of Go. Through trial and error, it was discovered that staticcheck
v0.2.2 is the latest version that is compatible with Go 1.16.

The authors of staticcheck also recommend pinning the version in CI
pipelines to prevent unintentional breakage of the build [1].

References
[1]: https://staticcheck.io/docs/running-staticcheck/ci/github-actions/#version

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
       - name: Get dependencies
         run: go get -t -v ./...
       - name: Install staticcheck
-        run: go install honnef.co/go/tools/cmd/staticcheck@latest
+        run: go install honnef.co/go/tools/cmd/staticcheck@v0.2.2
       - name: Lint
         run: staticcheck -checks=all ./...
       - name: Test