CI: test on gotip

Since the new linker was failing on our crypto/aes shenanigans until the
recent commit to remove it for literal obfuscation.

Building Go does take about two minutes on the CI machine, but that's
fast enough. One can see the exact version that was used via the 'go
version' line.

.github/workflows/test.yml

@@ -9,10 +9,29 @@ jobs:
     runs-on: ${{ matrix.platform }}
     steps:
     - name: Install Go
-      uses: actions/setup-go@v1
+      uses: actions/setup-go@v2
       with:
         go-version: ${{ matrix.go-version }}
     - name: Checkout code
       uses: actions/checkout@v2
     - name: Test
-      run: go test ./...
+      run: |
+        go version
+        go test ./...
+
+  test-gotip:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Install Go
+      run: |
+        git clone --depth=1 https://go.googlesource.com/go $HOME/gotip
+        cd $HOME/gotip/src
+        ./make.bash
+        echo "::set-env name=GOROOT::$HOME/gotip"
+        echo "::add-path::$HOME/gotip/bin"
+    - name: Checkout code
+      uses: actions/checkout@v2
+    - name: Test
+      run: |
+        go version
+        go test ./...