ci: Disable staticcheck on tip-of-tree compiler. (#664)

* ci: Disable staticcheck on tip-of-tree compiler.

staticcheck only supports the latest 2 copmiler releases.

Fixes #662.

* Set RUN_STATICCHCECK for subsequent steps.
diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 9b433c8..396a383 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -55,6 +55,7 @@
           cd $HOME/gotip/src
           ./make.bash
           echo "GOROOT=$HOME/gotip" >> $GITHUB_ENV
+          echo "RUN_STATICCHECK=false" >> $GITHUB_ENV
           echo "$HOME/gotip/bin:$PATH" >> $GITHUB_PATH
 
       - name: Checkout the repo
@@ -122,6 +123,7 @@
           cd $HOME/gotip/src
           ./make.bash
           echo "GOROOT=$HOME/gotip" >> $GITHUB_ENV
+          echo "RUN_STATICCHECK=false" >> $GITHUB_ENV
           echo "$HOME/gotip/bin" >> $GITHUB_PATH
 
       - name: Checkout the repo
diff --git a/test.sh b/test.sh
index 50b6524..0ecb53a 100755
--- a/test.sh
+++ b/test.sh
@@ -23,7 +23,9 @@
 # All packages.
 PKG=$(go list ./...)
 
-staticcheck $PKG
+if [ "$RUN_STATICCHECK" != "false" ]; then
+  staticcheck $PKG
+fi
 
 # Packages that have any tests.
 PKG=$(go list -f '{{if .TestGoFiles}} {{.ImportPath}} {{end}}' ./...)