Gentoo's sandbox does not whitelist this path by default yet.
TODO: bug link.

(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)

Until we have a sandbox whitelisting the path let's rely on ccache's default.
--- a/src/Config.cpp
+++ b/src/Config.cpp
@@ -885,11 +885,5 @@ Config::check_key_tables_consistency()
 std::string
 Config::default_temporary_dir(const std::string& cache_dir)
 {
-#ifdef HAVE_GETEUID
-  std::string user_tmp_dir = FMT("/run/user/{}", geteuid());
-  if (Stat::stat(user_tmp_dir).is_directory()) {
-    return user_tmp_dir + "/ccache-tmp";
-  }
-#endif
   return cache_dir + "/tmp";
 }