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.)

And see https://github.com/ccache/ccache/discussions/1086#discussioncomment-3327565.

https://bugs.gentoo.org/837362 for 4.6 issue.
https://bugs.gentoo.org/883799 and https://bugs.gentoo.org/887019 for 4.8 issue.
--- a/src/ccache/config.cpp
+++ b/src/ccache/config.cpp
@@ -1505,8 +1505,7 @@ Config::check_key_tables_consistency()
 fs::path
 Config::default_temporary_dir() const
 {
-  const auto dir = get_xdg_runtime_tmp_dir();
-  return !dir.empty() ? dir : m_cache_dir / "tmp";
+  return m_cache_dir / "tmp";
 }
 
 std::filesystem::path