chore: update terraform to 1.11.1 in nix image (#16880)

Followup PR to #16781, update the terraform version in our Nix devshell.

Additionally: 

1. Switches from DeterminateSystems/nix-installer-action to nixbuild/nix-quick-install-action -- quicker installer, reduces actions time from ~60 seconds to ~1 seconds.
2. Adds nix-community/cache-nix-action for better caching with garbage collection -- avoids unnecessary rebuilding on subsequent runs, reduces nix image build time from ~6 minutes to <4 minutes.
3. Adds nixpkgs-unstable input to use Terraform 1.11.1

Change-Id: I05d6dfd3f3cf1af48cf8a2d9e61b396bcd2b7191
Signed-off-by: Thomas Kosiewski <tk@coder.com>

flake.lock

@@ -44,11 +44,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1737885640,
-        "narHash": "sha256-GFzPxJzTd1rPIVD4IW+GwJlyGwBDV1Tj5FLYwDQQ9sM=",
+        "lastModified": 1741600792,
+        "narHash": "sha256-yfDy6chHcM7pXpMF4wycuuV+ILSTG486Z/vLx/Bdi6Y=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "4e96537f163fad24ed9eb317798a79afc85b51b7",
+        "rev": "ebe2788eafd539477f83775ef93c3c7e244421d3",
         "type": "github"
       },
       "original": {
@@ -74,6 +74,22 @@
         "type": "github"
       }
     },
+    "nixpkgs-unstable": {
+      "locked": {
+        "lastModified": 1741513245,
+        "narHash": "sha256-7rTAMNTY1xoBwz0h7ZMtEcd8LELk9R5TzBPoHuhNSCk=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "e3e32b642a31e6714ec1b712de8c91a3352ce7e1",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "pnpm2nix": {
       "inputs": {
         "flake-utils": [
@@ -103,6 +119,7 @@
         "flake-utils": "flake-utils",
         "nixpkgs": "nixpkgs",
         "nixpkgs-pinned": "nixpkgs-pinned",
+        "nixpkgs-unstable": "nixpkgs-unstable",
         "pnpm2nix": "pnpm2nix"
       }
     },