CI: start testing on Go 1.17.x

Also bump the gotip commit to the first after the merge window was open,
which simply bumps the internal version from 1.17 to 1.18.
We're leaving GO_COMMIT there for now,
as the 1.18 merge window is currently very noisy with generics changes.

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.16.x]
+        go-version: [1.16.x, 1.17.x]
         platform: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.platform }}
     steps:
@@ -31,15 +31,15 @@ jobs:
     steps:
     - name: Install Go
       env:
-        GO_COMMIT: 3e48c0381fd1beb78e993e940c3b46ca9898ce6d # 2021-07-21
+        GO_COMMIT: 46fd547d899286982971474b329d7a95da4f2a6b # 2021-08-11
       run: |
         cd $HOME
         mkdir $HOME/gotip
         cd $HOME/gotip
 
         wget -O gotip.tar.gz https://go.googlesource.com/go/+archive/${GO_COMMIT}.tar.gz
         tar -xf gotip.tar.gz
-        echo "devel go1.17-${GO_COMMIT}" >VERSION
+        echo "devel go1.18-${GO_COMMIT}" >VERSION
 
         cd src
         ./make.bash