Revert "tests: remove obsolete code to create custom build direc..."

Revert submission 3850

Reason for revert: caused multiple regressions noticed in https://git.lix.systems/lix-project/lix/issues/975 and https://git.lix.systems/lix-project/lix/issues/966 (suspected).

Root cause analysis has not been done yet and this breaks Lix on Darwin on HEAD.

Reverted changes: /q/submissionid:3850

Change-Id: I56f871786760eb9f54c02773f0617de9878c1382

tests/functional/check.sh

@@ -37,6 +37,9 @@ if checkBuildTempDirRemoved $TEST_ROOT/log; then false; fi
 test_custom_build_dir() {
   local customBuildDir="$TEST_ROOT/custom-build-dir"
 
+  # Nix does not create the parent directories, and perhaps it shouldn't try to
+  # decide the permissions of build-dir.
+  mkdir "$customBuildDir"
   nix-build check.nix -A failed --argstr checkBuildId $checkBuildId \
       --no-out-link --keep-failed --option build-dir "$TEST_ROOT/custom-build-dir" 2> $TEST_ROOT/log || status=$?
   [ "$status" = "100" ]