go.toolchain.rev: use new statically built toolchain

Also removes the toolchain builds from flake.nix. For now the flake
build uses upstream Go 1.20, a followup change will switch it back to
our custom toolchain.

Updates tailscale/corp#9005

Signed-off-by: David Anderson <danderson@tailscale.com>

pull-toolchain.sh

@@ -12,6 +12,6 @@ if [ "$upstream" != "$current" ]; then
 	./update-flake.sh
 fi
 
-if [ -n "$(git diff-index --name-only HEAD -- go.toolchain.rev go.toolchain.sri go.mod.sri)" ]; then
+if [ -n "$(git diff-index --name-only HEAD -- go.toolchain.rev go.mod.sri)" ]; then
     echo "pull-toolchain.sh: changes imported. Use git commit to make them permanent." >&2
 fi