Update Bazel pin, ignore build-* directories (#2461)

Updates the Bazel pin to 8.1.0 so that build-* directories can be
properly ignored.

.bazelignore

@@ -1,4 +0,0 @@
-# Don't accidentally pick up external CMake deps with Bazel build files.
-build
-# Don't treat submodules as part of this project.
-lib

.bazelrc

@@ -4,3 +4,7 @@ common --host_per_file_copt=external/.*@-w
 
 # Produce useful output when the build fails.
 common --verbose_failures
+
+# Silence warnings about old bazel_dep pins. Bazel 8 brings along newer ones,
+# but we can support Bazel 7 (for now).
+common --check_direct_dependencies=off

.bazelversion

@@ -1 +1 @@
-7.2.1
+8.1.0

REPO.bazel

@@ -0,0 +1,10 @@
+ignore_directories(
+    [
+        # Don't accidentally pick up external CMake deps with Bazel build files.
+        "cmake-*",
+        "build",
+        "build-*",
+        # Don't treat submodules as part of this project.
+        "lib",
+    ],
+)