Repair wp/5.8 branch Actions: single Node.js version testing - #168

The `checks` job tested against Node `12` and `14`. `wp/5.8` is a
frozen branch that will never move off the Node version pinned in
`.nvmrc` (`14`), so testing an older, unsupported version adds nothing
and only costs CI time. Drop the matrix entirely and let
`actions/setup-node` read the version from `.nvmrc` directly, so the
two stay in sync by construction instead of by convention.

The `admin` job's matrix combined a `part` dimension (test-file
sharding across 4 parallel jobs) with a `node: ['14']` dimension that
only ever had one value. `wp/5.8` is a frozen branch that will never
move off the Node version pinned in `.nvmrc` (`14`), so a version
matrix dimension here was never providing coverage, only overhead.
Drop the `node` dimension and let `actions/setup-node` read the
version from `.nvmrc` directly, while keeping the `part` dimension
that does real sharding work.

The `build` job's matrix had a single `node: ['14']` dimension. `wp/5.8`
is a frozen branch that will never move off the Node version pinned
in `.nvmrc` (`14`), so a version matrix here was never providing
coverage. Drop the matrix entirely and let `actions/setup-node` read
the version from `.nvmrc` directly.