Optimize Go version matrix strategy for CI workflow (#17)

Before the CI workflow used a matrix strategy to run the `lint-go` and
`test` jobs, but this was improved to make the workflow run faster by
avoiding unnecessary steps:

- The `lint-go` job has been changed to only run on the currently latest
  stable Go version `1.15.x` [1] only on Linux because `golangci-lint`
  doesn't care about the Go version and OS it runs on but only
  statically checks the source code.
- The `test` job has been changed to only run on the currently latest
  stable Go version `1.15.x` [1].

These changes also help to keep the required GitHub Action run minutes
for the account of this repository as small as possible without wasting
resources for unnecessary tasks.

[1]: https://golang.org/doc/go1.15

Resolves GH-16

.github/workflows/ci.yml

@@ -27,13 +27,7 @@ jobs:
       - name: Run linters
         run: yarn lint
   lint-go:
-    strategy:
-      matrix:
-        # Use the official Go version policy and run for the latest three major version releases.
-        # See https://golang.org/doc/devel/release.html#policy for more details.
-        go-version: [1.13.x, 1.14.x, 1.15.x]
-        os: [macos-latest, ubuntu-latest, windows-latest]
-    runs-on: ${{ matrix.os }}
+    runs-on: ubuntu-latest
     steps:
       - name: Print metadata and context information
         run: |
@@ -42,6 +36,10 @@ jobs:
           echo "Workflow Actor: $GITHUB_ACTOR"
       - name: Checkout repository
         uses: actions/checkout@v2
+      - name: Install Go
+        uses: actions/setup-go@v2
+        with:
+          go-version: "1.15.x"
       - name: "Run golangci-lint"
         # The official golangci-lint action created and maintained by the golangci-lint project.
         # See https://github.com/golangci/golangci-lint-action for more details.
@@ -51,9 +49,7 @@ jobs:
   test:
     strategy:
       matrix:
-        # Use the official Go version policy and run for the latest three major version releases.
-        # See https://golang.org/doc/devel/release.html#policy for more details.
-        go-version: [1.13.x, 1.14.x, 1.15.x]
+        go-version: ["1.15.x"]
         os: [macos-latest, ubuntu-latest, windows-latest]
     runs-on: ${{ matrix.os }}
     needs: