link_tree.py: format conflict error message (#29870)

Show each `[src a] and [src b] both project to [dst]` on a separate
line.

lib/spack/llnl/util/link_tree.py

@@ -426,9 +426,9 @@ def __init__(self, conflicts):
         A human-readable summary of file system view merge conflicts (showing only the
         first 3 issues.)
         """
-        msg = "{0} fatal error(s) when merging prefixes:\n".format(len(conflicts))
+        msg = "{0} fatal error(s) when merging prefixes:".format(len(conflicts))
         # show the first 3 merge conflicts.
         for conflict in conflicts[:3]:
-            msg += "    `{0}` and `{1}` both project to `{2}`".format(
+            msg += "\n    `{0}` and `{1}` both project to `{2}`".format(
                 conflict.src_a, conflict.src_b, conflict.dst)
         super(MergeConflictSummary, self).__init__(msg)