Override ccacheWrapper to use the correct dir
diff --git a/src/overlays/ccache.nix b/src/overlays/ccache.nix
new file mode 100644
index 0000000..c752df4
--- /dev/null
+++ b/src/overlays/ccache.nix
@@ -0,0 +1,9 @@
+final: prev: {
+    ccacheWrapper = prev.ccacheWrapper.override {
+        extraConfig = ''
+            export CCACHE_COMPRESS=1
+            export CCACHE_DIR=/var/cache/ccache
+            export CCACHE_UMASK=007
+        '';
+    };
+}