fix(ops/machines/build01): put Nix builds temp dir in /var/tmp

This solves issues such as
```
cargo:warning=Fatal error: can't create
/build/source/target/debug/build/zstd-sys-fa4cde6de82f89a8/out/88f362f13b0528ed-zstd_decompress_block.o:
No space left on device
```

on the Buildkite CI.

Fixes #82.

Change-Id: Iee9516d8d595b718824c3e7b28c01c3ef9e9d090
Signed-off-by: Raito Bezarius <raito@lix.systems>
Reviewed-on: https://cl.snix.dev/c/snix/+/30143
Autosubmit: Ryan Lahfa <masterancpp@gmail.com>
Tested-by: besadii
Reviewed-by: Florian Klink <flokli@flokli.de>

ops/machines/build01/default.nix

@@ -27,6 +27,12 @@ in
     };
   };
 
+  # Put builds in /var/tmp as it can be quite big and would cause spurious
+  # failures from time to time: https://git.snix.dev/snix/snix/issues/82
+  systemd.services.nix-daemon.serviceConfig.Environment = [
+    "TMPDIR=/var/tmp"
+  ];
+
   services.depot.buildkite = {
     enable = true;
     agentCount = 32;