Non-determinisitic behavior with `import` util

I noticed a build failure on https://github.com/bazelbuild/rules_rust/pull/1669 which I didn't expect based on the changes being made:

https://buildkite.com/bazel/rules-rust-rustlang/builds/7484#0184af2d-b9bf-42d9-afe6-12b999c35d61
```
Executing tests from //util/import:import_internal_test
-----------------------------------------------------------------------------

running 10 tests
test tests::test_label_parsing ... ok
test tests::test_macro_input_parsing_errors ... ok
test tests::test_expansion_failures ... ok
test tests::test_expand_imports_without_renaming ... ok
test tests::test_decode ... ok
test tests::test_encode ... ok
test tests::test_expand_imports_with_renaming ... ok
test tests::test_substitutions_compose ... ok
test tests::composition_is_identity ... FAILED
test tests::test_substitutions_concatenate ... ok

failures:

---- tests::composition_is_identity stdout ----
thread 'tests::composition_is_identity' panicked at '[quickcheck] TEST FAILED. Arguments: ("_x ")', external/rules_rust_util_import__quickcheck-1.0.3/src\tester.rs:165:28
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

failures:
tests::composition_is_identity

test result: FAILED. 9 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.09s
```

@hlopko @scentini would either of you have any idea what's happening there?