fix: harden Bazel RBE follow-up

[haasonsaas]

Summary

• include MODULE.bazel.lock in the Bazel RBE push path filter
• fall back to a usable bazel binary when Go is present but GOPATH/bin/bazelisk is absent
• resolve bazelisk from every GOPATH entry before falling back

Internal source PR: evalops/maestro-internal#1973

Verification

• git diff --check
• ruby YAML.load_file(.github/workflows/bazel-rbe.yml)
• make -n bazel-test BAZEL=/bin/echo
• fake multi-GOPATH smoke resolving bazelisk from the second GOPATH entry

[cursor]

PR Summary

Low Risk
Low risk CI/tooling change: adjusts the Bazel RBE workflow trigger paths and makes Makefile Bazel discovery more robust when Go is installed but bazelisk isn’t on GOPATH/bin.

Overview
Ensures the Bazel RBE GitHub Actions workflow reruns when MODULE.bazel.lock changes by adding it to the push paths filter.

Hardens Makefile Bazel resolution so $(BAZEL) falls back to an existing bazel on PATH (and correctly handles multi-entry GOPATH) when bazelisk isn’t available.

^{Reviewed by Cursor Bugbot for commit d15e329. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

This PR changes mirrored Maestro source files in the public repo, but it does not link the matching private source-of-truth PR.

Add one of these to the PR body, then re-run the check:

• https://github.com/evalops/maestro-internal/pull/<number>
• evalops/maestro-internal#<number>
• maestro-internal#<number>

Mirrored files touched:

• Makefile

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