flake.nix: update Nix to use tailscale/go 1.25.2 (#17500)

Update Nix flake to use go 1.25.2
Create the hash from the toolchain rev file automatically from
update-flake.sh

Updates tailscale/go#135

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

pull-toolchain.sh

@@ -11,6 +11,10 @@ if [ "$upstream" != "$current" ]; then
 	echo "$upstream" >go.toolchain.rev
 fi
 
-if [ -n "$(git diff-index --name-only HEAD -- go.toolchain.rev)" ]; then
+./tool/go version 2>/dev/null | awk '{print $3}' | sed 's/^go//' > go.toolchain.version
+
+./update-flake.sh
+
+if [ -n "$(git diff-index --name-only HEAD -- go.toolchain.rev go.toolchain.rev.sri go.toolchain.version)" ]; then
     echo "pull-toolchain.sh: changes imported. Use git commit to make them permanent." >&2
 fi