ci: pin the Node 26 matrix entry to 26.7 - #3027

Node 26.8.0 rewrote fs.readFile (nodejs/node#65327) to do open, fstat,
read and close in a single thread pool round trip, so binding.open is no
longer called from JS. mock-fs recovers the ReadFileContext prototype by
intercepting binding.open during a dummy readFile, so it now gets
undefined and throws at require time:

    TypeError: Cannot read properties of undefined (reading 'read')
        at exports.patchReadFileContext (mock-fs/lib/readfilecontext.js:40:30)

Because it throws on require rather than in a test, it takes out
config_test.ts and file_auth_test.ts in full.

The matrix entries are floating majors, so setup-node resolves '26' to
whatever the newest 26.x is at run time. That is why main went red on
the merge of kubernetes-client#3022 without any change to the code under test: the branch
last ran CI on 26.7.0, and by the time it merged five days later the
runner had picked up 26.8.1.

Pinning to 26.7 restores a green build. It is a stopgap: mock-fs has had
no functional release since February 2025 and the upstream report
(tschaub/mock-fs#447) is unanswered, so the durable fix is to stop
depending on it.