BAZEL_STABLE_STATUS_FILE and BAZEL_VOLATILE_STATUS_FILE should contain absolute paths

[matthewjh]

Currently BAZEL_STABLE_STATUS_FILE and BAZEL_VOLTAILE_STATUS_FILE are set in the env for stamp-aware build actions like run_binary. The trouble, these are set to relative paths from the execroot e.g. bazel-out/volatile-status.txt, which is not so helpful if..

• Using rules_js, where build actions run in the BINDIR and not the execroot
• Using rules with custom chdir settings

To eliminate the need for any path foo, it would be more helpful if these provided absolute filepaths that could be read as-is regardless of context.