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>

.github/workflows/dogfood.yaml

@@ -35,7 +35,26 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
 
       - name: Setup Nix
-        uses: DeterminateSystems/nix-installer-action@e50d5f73bfe71c2dd0aa4218de8f4afa59f8f81d # v16
+        uses: nixbuild/nix-quick-install-action@5bb6a3b3abe66fd09bbf250dce8ada94f856a703 # v30
+
+      - uses: nix-community/cache-nix-action@aee88ae5efbbeb38ac5d9862ecbebdb404a19e69 # v6.1.1
+        with:
+          # restore and save a cache using this key
+          primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
+          # if there's no cache hit, restore a cache by this prefix
+          restore-prefixes-first-match: nix-${{ runner.os }}-
+          # collect garbage until Nix store size (in bytes) is at most this number
+          # before trying to save a new cache
+          # 1G = 1073741824
+          gc-max-store-size-linux: 5G
+          # do purge caches
+          purge: true
+          # purge all versions of the cache
+          purge-prefixes: nix-${{ runner.os }}-
+          # created more than this number of seconds ago relative to the start of the `Post Restore` phase
+          purge-created: 0
+          # except the version with the `primary-key`, if it exists
+          purge-primary-key: never
 
       - name: Get branch name
         id: branch-name