files: avoid surprises when linking files

In the unlikely but possible event that a directory is created in
place of a target file *after* `checkLinkTargets` but *before*
`linkGeneration`, currently the link is created *inside* the directory,
which is surprising.

To avoid this, use `ln -T` and bail if it fails.

Ideally, `checkLinkTargets` and `linkGeneration` should be more tightly
coupled.

modules/files.nix

@@ -185,8 +185,9 @@ in
               $VERBOSE_ECHO "Skipping '$targetPath' as it is identical to '$sourcePath'"
             else
               # Place that symlink, --force
+              # This can still fail if the target is a directory, in which case we bail out.
               $DRY_RUN_CMD mkdir -p $VERBOSE_ARG "$(dirname "$targetPath")"
-              $DRY_RUN_CMD ln -nsf $VERBOSE_ARG "$sourcePath" "$targetPath"
+              $DRY_RUN_CMD ln -Tsf $VERBOSE_ARG "$sourcePath" "$targetPath" || exit 1
             fi
           done
         '';