chore: fix c8 dependency breakage on node <= 18

[pearigee]

The Issue

The error blocking release:

 ERR_PNPM_UNSUPPORTED_ENGINE  Unsupported environment (bad pnpm and/or Node.js version)

This error happened while installing the dependencies of c8@10.1.3
 at test-exclude@7.0.2
 at minimatch@10.2.5

Your Node version is incompatible with "brace-expansion@5.0.8".

Expected version: 20 || >=22
Got: v18.20.8

This is happening because the package's manifest has an engines.node field specified.
To fix this issue, install the required Node version.

---

The Fix

Here is the exact dependency chain that triggered the failure:

c8@10.1.3
  └── test-exclude@7.0.2
      └── minimatch@10.2.5
          └── brace-expansion@5.0.8 (requires node: "20 || >=22")

1. c8@10.1.3 pulled in test-exclude@7.0.2.
2. test-exclude@7.0.2 updated its dependency to minimatch@10.2.5.
3. minimatch@10.2.5 pulled in brace-expansion@5.0.8.
4. brace-expansion@5.0.8 strictly requires Node 20 || >=22 in its engines specification, breaking installation under Node 18.

As a result, we need to pin to an older version of C8 for versions of Node older than 18. Rather that implementing a global override, updating the .pnpmfile.cjs enables us to do a targeted override ONLY for Node 18. That means that as we upgrade to Node 22, every other package can continue to use 10+ like they do currently.

---

Why this works

Note, this works because in our CI we explicitly set the PNPM file on install: https://github.com/googleapis/google-cloud-node/blob/main/ci/run_single_test.sh#L45

[gemini-code-assist]

Code Review

This pull request introduces a workaround in .pnpmfile.cjs to override the c8 dependency to ^9.1.0 when running on Node.js versions 18 or below, ensuring CI compatibility. The review feedback suggests using context.log instead of console.log to prevent stdout pollution and recommends simplifying the Node.js major version extraction by parsing process.versions.node directly.

[pearigee]

This contributes to unblocking #8904 .