Install gox before checking out code

Otherwise newer Go versions complain that the hash for the installed
version of gox is not in the go.mod, which we don't want anyways because
the tests should use the latest version of gox.

.github/workflows/tests.yml

@@ -233,12 +233,12 @@ jobs:
         with:
           go-version: ${{ env.latest_go }}
 
-      - name: Check out code
-        uses: actions/checkout@v2
-
       - name: Install gox
         run: |
-          go install github.com/mitchellh/gox
+          go install github.com/mitchellh/gox@latest
+
+      - name: Check out code
+        uses: actions/checkout@v2
 
       - name: Cross-compile with gox for ${{ matrix.targets }}
         env: