CI: add Go 1.20rc3, disable gotip for now

tip is now the start of Go 1.21 as of a couple of days ago.
However, the first week or two is when the biggest changes land,
which means that Go tip is far more prone to bugs and changes that might
break garble.

We'll start tracking tip again in a few weeks, once the dust has settled
and we can look at what changes might have broken garble.

.github/workflows/test.yml

@@ -22,7 +22,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.19.x]
+        go-version: [1.19.x, 1.20.0-rc.3]
         # TODO: revert to macos-latest once we figure out https://github.com/burrowers/garble/issues/609.
         os: [ubuntu-latest, macos-11, windows-latest]
     runs-on: ${{ matrix.os }}
@@ -80,6 +80,7 @@ jobs:
       run: go test -timeout=15m ./...
 
   test-gotip:
+    if: false # let tip for 1.21 settle a bit
     runs-on: ubuntu-latest
     steps:
     - name: Install Go