Build on bare bare metal servers

* Allow builds to use cached Nix packages
* Prevent them from running on my VPS
* Possibly avoid re-downloading the SVN every single time?

.github/workflows/build.yml

@@ -9,7 +9,7 @@
 
 jobs:
   build:
-    runs-on: nix-latest
+    runs-on: bare-metal
     strategy:
       matrix:
         arch:

.github/workflows/pr.yml

@@ -3,7 +3,7 @@
 
 jobs:
   check:
-    runs-on: nix-latest
+    runs-on: bare-metal
     steps:
       - uses: actions/checkout@v6
       - name: Flake check