Do not fail if $HOME/.ccache is missing (ocaml#3957)

bwrap/osx: do not fail if $HOME/.ccache is missing: check for directory existence in `add_mounts`

See https://discuss.ocaml.org/t/dune-1-11-1-compilation-failed/4248

This was already present in bwrap, so only sandbox needs to be updated

Signed-off-by: Marcello Seri <marcello.seri@gmail.com>

src/state/shellscripts/sandbox_exec.sh

@@ -6,11 +6,13 @@ POL="$POL"'(allow network* (remote unix))'
 POL="$POL"'(allow file-write* (literal "/dev/null") (literal "/dev/dtracehelper"))'
 
 add_mounts() {
-    local DIR="$(cd "$2" && pwd -P)"
-    case "$1" in
-        ro) POL="$POL"'(deny file-write* (subpath "'"$DIR"'"))';;
-        rw) POL="$POL"'(allow file-write* (subpath "'"$DIR"'"))';;
-    esac
+    if [ -d "$2" ]; then
+      local DIR="$(cd "$2" && pwd -P)"
+      case "$1" in
+          ro) POL="$POL"'(deny file-write* (subpath "'"$DIR"'"))';;
+          rw) POL="$POL"'(allow file-write* (subpath "'"$DIR"'"))';;
+      esac
+    fi
 }
 
 if [ -z ${TMPDIR+x} ]; then