Gentoo's sandbox does not whitelist this path by default yet.

(4.1 update:
https://github.com/ccache/ccache/issues/984
https://github.com/ccache/ccache/issues/1044
https://github.com/ccache/ccache/commit/a0edd4294f6a5a2d3f0c7b01273736f975f250e1
https://github.com/ccache/ccache/commit/ef2e922f9642f943199138447b29ec53fa63ea68
... gets us closer, but not there yet.)

https://bugs.gentoo.org/837362 for 4.6 issue.
--- a/src/Config.cpp
+++ b/src/Config.cpp
@@ -1031,14 +1031,5 @@ Config::check_key_tables_consistency()
 std::string
 Config::default_temporary_dir(const std::string& cache_dir)
 {
-  static const std::string run_user_tmp_dir = [] {
-#ifdef HAVE_GETEUID
-    auto dir = FMT("/run/user/{}/ccache-tmp", geteuid());
-    if (Util::create_dir(dir)) {
-      return dir;
-    }
-#endif
-    return std::string();
-  }();
-  return !run_user_tmp_dir.empty() ? run_user_tmp_dir : cache_dir + "/tmp";
+  return cache_dir + "/tmp";
 }