.github/Makefile/flake: update nix flake support (#16636)

Cleanup nix support, make flake easier to read with nix-systems.
This also harmonizes with golinks flake setup and reduces an input
dependency by 1.

Update deps test to ensure the vendor hash stays harmonized
with go.mod.

Update make tidy to ensure vendor hash stays current.

Overlay the current version of golang, tailscale runs
recent releases faster than nixpkgs can update them into
the unstable branch.

Updates #16637

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

.github/workflows/test.yml

@@ -720,10 +720,10 @@ jobs:
     - name: check that 'go mod tidy' is clean
       working-directory: src
       run: |
-        ./tool/go mod tidy
+        make tidy
         echo
         echo
-        git diff --name-only --exit-code || (echo "Please run 'go mod tidy'."; exit 1)
+        git diff --name-only --exit-code || (echo "Please run 'make tidy'"; exit 1)
 
   licenses:
     runs-on: ubuntu-24.04