CI: start testing on Go 1.18rc1, bump gotip to 1.19

Our upcoming release will focus on supporting 1.17.x and 1.18.x.
Since gotip is now merging features for the future 1.19.x,
let's add 1.18rc1 to the matrix and bump gotip.

This will make our CI jobs a bit slower for the time being,
but that's the price to pay to ensure we support both versions.
After the next garble version is released,
we can consider dropping support for 1.17.x.

.github/workflows/test.yml

@@ -22,7 +22,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.17.x]
+        go-version: [1.17.x, 1.18.0-rc.1]
         os: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.os }}
     steps:
@@ -71,7 +71,7 @@ jobs:
     steps:
     - name: Install Go
       env:
-        GO_COMMIT: e50f0f372b07149e9cf16b8fec80d2d72efe2a87 # 2022-02-11
+        GO_COMMIT: 86b5f6a7be707b9d84108df6f459c7e84bf9dbac # 2022-03-02
       run: |
         cd $HOME
         mkdir $HOME/gotip
@@ -79,7 +79,7 @@ jobs:
 
         wget -O gotip.tar.gz https://go.googlesource.com/go/+archive/${GO_COMMIT}.tar.gz
         tar -xf gotip.tar.gz
-        echo "devel go1.18-${GO_COMMIT}" >VERSION
+        echo "devel go1.19-${GO_COMMIT}" >VERSION
 
         cd src
         # GOGC=off helps Go build about 20% faster, if we can spare memory.