Look up .build-id files relative to the directories in the
colon-separated environment variable NIX_DEBUG_INFO_DIRS, unless
overriden by --debuginfo-path.

diff -ru elfutils-0.169-orig/libdwfl/argp-std.c elfutils-0.169/libdwfl/argp-std.c
--- elfutils-0.169-orig/libdwfl/argp-std.c	2017-05-02 23:05:52.000000000 +0200
+++ elfutils-0.169/libdwfl/argp-std.c	2017-07-28 16:08:06.739558106 +0200
@@ -376,5 +376,7 @@
 const struct argp *
 dwfl_standard_argp (void)
 {
+  debuginfo_path = getenv("NIX_DEBUG_INFO_DIRS");
+
   return &libdwfl_argp;
 }
diff -ru elfutils-0.169-orig/src/stack.c elfutils-0.169/src/stack.c
--- elfutils-0.169-orig/src/stack.c	2017-02-24 11:55:28.000000000 +0100
+++ elfutils-0.169/src/stack.c	2017-07-28 15:50:06.743196696 +0200
@@ -631,6 +631,8 @@
   /* Set locale.  */
   (void) setlocale (LC_ALL, "");
 
+  debuginfo_path = getenv("NIX_DEBUG_INFO_DIRS");
+
   const struct argp_option options[] =
     {
       { NULL, 0, NULL, 0, N_("Input selection options:"), 0 },