setup go toolchain for unit test workflow

i think we were relying on the go toolchain available inside the runner
image, which could be out of date. Instead we should use the setup-go
action which should have the latest version of the toolchain

Signed-off-by: Bohan Chen <[email protected]>

.github/workflows/unit-test.yaml

@@ -10,6 +10,10 @@ jobs:
     - uses: actions/checkout@v3
       with:
         fetch-depth: 0
+    - name: Set up Go
+      uses: actions/setup-go@v4
+      with:
+        go-version-file: 'go.mod'
     - name: Run tests
       uses: ./.github/actions/run-tests
     - name: Report coverage