nix: update nixpkgs flake, override go_1_20 for tailscale_go (#7139)

Bleeding edge Tailscale Nix flake broke after updating to go1.20rc3.

Go 1.20 moved to Go 1.17 as a bootstarp toolchain. Fortunately nixpkgs
nixos-unstable already had a 1.20.nix with bootstrap117.nix.

```
❯ ./result/bin/tailscale version
1.37.0-dev
  track: unstable (dev); frequent updates and bugs are likely
  go version: go1.20rc3-ts6a17f14c05
```

Signed-off-by: Shayne Sweeney <shayne@tailscale.com>

flake.lock

@@ -33,11 +33,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1671848398,
-        "narHash": "sha256-cJIIPd1kvCI6ne/S0facbiBNH7sZUzk405GfdSJPwZE=",
+        "lastModified": 1675153841,
+        "narHash": "sha256-EWvU3DLq+4dbJiukfhS7r6sWZyJikgXn6kNl7eHljW8=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "bb0359be0a1a08c8d74412fe8c69aa2ffb3f477e",
+        "rev": "ea692c2ad1afd6384e171eabef4f0887d2b882d3",
         "type": "github"
       },
       "original": {