Fix proto_cross_repo_boundary build_file attribute

Bazel 7 enforces that this be a proper target label, not a relative
path.

```txt
ERROR: WORKSPACE:91:37: fetching new_local_repository rule
  //external:proto_cross_repo_boundary:
  .../external/test/proto_cross_repo_boundary/repo/BUILD.repo
  is not a regular file; if you're using a relative or absolute path for
  `build_file` in your `new_local_repository` rule, please switch to
  using a label instead

INFO: Repository remote_jdk8_macos instantiated at:
  WORKSPACE:175:18: in <toplevel>
  .../external/rules_java/java/repositories.bzl:120:10:
    in remote_jdk8_repos
  .../external/bazel_tools/tools/build_defs/repo/utils.bzl:240:18:
    in maybe
  .../external/rules_java/toolchains/remote_java_repository.bzl:48:17:
    in remote_java_repository

Repository rule http_archive defined at:
  .../external/bazel_tools/tools/build_defs/repo/http.bzl:384:31:
  in <toplevel>

ERROR: no such package '@@proto_cross_repo_boundary//':
  .../external/test/proto_cross_repo_boundary/repo/BUILD.repo
  is not a regular file; if you're using a relative or absolute path for
  `build_file` in your `new_local_repository` rule, please switch to
  using a label instead

ERROR: test/proto_cross_repo_boundary/BUILD:3:22:
  //test/proto_cross_repo_boundary:sample_scala_proto depends on
  @@proto_cross_repo_boundary//:sample_proto in repository
  @@proto_cross_repo_boundary which failed to fetch. no such package
  '@@proto_cross_repo_boundary//':
  .../external/test/proto_cross_repo_boundary/repo/BUILD.repo
  is not a regular file; if you're using a relative or absolute path for
  `build_file` in your `new_local_repository` rule, please switch to
  using a label instead

Use --verbose_failures to see the command lines of failed build steps.
ERROR: Analysis of target
  '//test/proto_cross_repo_boundary:sample_scala_proto' failed;
  build aborted: Analysis failed
```

Author: mbland

test/proto_cross_repo_boundary/repo.bzl

@@ -2,5 +2,5 @@ def proto_cross_repo_boundary_repository():
     native.new_local_repository(
         name = "proto_cross_repo_boundary",
         path = "test/proto_cross_repo_boundary/repo",
-        build_file = "test/proto_cross_repo_boundary/repo/BUILD.repo",
+        build_file = "//test/proto_cross_repo_boundary:repo/BUILD.repo",
     )