c++: fall back to cache dir in /tmp

rules_go is making calls that do `env - <...>` and are setting neither
TMPDIR nor HOME. So we should always fallback somewhere.

toolchain/defs.bzl

@@ -81,11 +81,14 @@ set -e
 
 if [[ -n "$TMPDIR" ]]; then
     _cache_prefix=$TMPDIR
-else
-  _cache_prefix="$HOME/.cache"
+elif [[ -n "$HOME" ]]; then
     if [[ "$(uname)" = Darwin ]]; then
         _cache_prefix="$HOME/Library/Caches"
+    else
+        _cache_prefix="$HOME/.cache"
     fi
+else
+    _cache_prefix=/tmp
 fi
 
 export ZIG_LOCAL_CACHE_DIR="$_cache_prefix/bazel-zig-cc"