fix(builtin): fix linker issue when running test with "local" tag on osx & linux

[gregmagolan]

    const maybe = path.resolve(`${symlinkRoot}/external/${root}`);
    if (fs.existsSync(maybe)) {
      // Under runfiles, when not in the sandbox we must symlink node_modules down at the execroot
      // `execroot/my_wksp/external/npm/node_modules` since `runfiles/npm/node_modules` will be a
      // directory and not a symlink back to the root node_modules where we expect
      // to resolve from. This case is tested in internal/linker/test/local.
      return maybe;
    }
    // However, when in the sandbox, `execroot/my_wksp/external/npm/node_modules` does not exist,
    // so we must symlink into `runfiles/npm/node_modules`. This directory exists whether legacy
    // external runfiles are on or off.
    return path.resolve(`${startCwd}/../${root}`)

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What is the current behavior?

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Does this PR introduce a breaking change?

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