[Fix][CI] grpc_bazel_rbe_nonbazel job: align kokoro and bazel timeouts (#41231)

[Fix][CI] grpc_bazel_rbe_nonbazel job: align kokoro and bazel timeouts (#41231)

Target `//tools/bazelify_tests/test:cpp_distribtest_cmake_aarch64_cross_linux` seems to go over Bazel's `--test_timeout` limit from time to time.

Bazel `--test_timeout` flag was initially introduced in #38123 and set to 30 minutes below Kokoro's job `timeout_mins`. Since then, we've increased Kokoro's timeout several times without making corresponding changes to bazel's `test_timeout`.

This PR updates Bazel's test timeout to aligned with Kokoro's job timeout, and adds a reminder to keep those in sync. In addition, I've broken `BAZEL_FLAGS` in multiple lines for better readability, proto text format spec [allows it](https://protobuf.dev/reference/protobuf/textformat-spec/#string).

Closes #41231

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/41231 from sergiitk:fix/ci/rbe-nonbazel-bazel-test-timeout 22970a18ce2768693292f566f31dd765916fbca0
PiperOrigin-RevId: 843999341