refactor: replace Terraform binary with OpenTofu

OpenTofu is a drop-in replacement for Terraform, so aside from changing
the binary, it only needs some minor adjustments.

flake.lock

@@ -5,11 +5,11 @@
         "systems": "systems"
       },
       "locked": {
-        "lastModified": 1710146030,
-        "narHash": "sha256-SZ5L6eA7HJ/nmkzGG7/ISclqe6oZdOZTNoesiInkXPQ=",
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
         "owner": "numtide",
         "repo": "flake-utils",
-        "rev": "b1d9ab70662946ef0850d488da1c9019f3a9752a",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
         "type": "github"
       },
       "original": {
@@ -20,11 +20,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1725001927,
-        "narHash": "sha256-eV+63gK0Mp7ygCR0Oy4yIYSNcum2VQwnZamHxYTNi+M=",
+        "lastModified": 1731797254,
+        "narHash": "sha256-df3dJApLPhd11AlueuoN0Q4fHo/hagP75LlM5K1sz9g=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "6e99f2a27d600612004fbd2c3282d614bfee6421",
+        "rev": "e8c38b73aeb218e27163376a2d617e61a2ad9b59",
         "type": "github"
       },
       "original": {