Use tmpfs in /tmp

The /tmp directory was using the SSD disk which is not erased across
boots. Nix will use /tmp to perform the builds, so we want it to be as
fast as possible. In general, all the machines have enough space to
handle large builds like LLVM.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>

m/common/fs.nix

@@ -25,4 +25,7 @@
     device = "none";
     fsType = "tracefs";
   };
+
+  # Mount a tmpfs into /tmp
+  boot.tmp.useTmpfs = true;
 }