nixos/systemd.tmpfiles.packages: fix an edge case

symlinkJoin can break (silently) when the passed paths contain symlinks
to directories.  This should work now.

Down-side: when lib/tmpfiles.d doesn't exist for some passed package,
the error message is a little less explicit, because we never get
to the postBuild phase (and symlinkJoin doesn't provide a better way):
/nix/store/HASH-NAME/lib/tmpfiles.d: No such file or directory

Also, it seemed pointless to create symlinks for whole package trees
and using only a part of the result (usually very small part).

1 files changed, 2 insertions, 0 deletions

diff --git a/pkgs/build-support/trivial-builders.nix b/pkgs/build-support/trivial-builders.nix
index 3c81a4ece657..eab5366e1839 100644
--- a/pkgs/build-support/trivial-builders.nix
+++ b/pkgs/build-support/trivial-builders.nix
@@ -240,6 +240,8 @@ rec {
    * This creates a single derivation that replicates the directory structure
    * of all the input paths.
    *
+   * BEWARE: it may not "work right" when the passed paths contain symlinks to directories.
+   *
    * Examples:
    * # adds symlinks of hello to current build.
    * symlinkJoin { name = "myhello"; paths = [ pkgs.hello ]; }