ci: Apply matrix env vars in the workflow, not in custom actions

[krlmlr]

Problem

Matrix entries in .github/versions-matrix.R can carry a generic env field
(one KEY=VALUE per line).
Until now, the rcc-full job only forwarded that field
to the custom before-install action as an input:

- uses: ./.github/workflows/custom/before-install
  with:
    env: ${{ matrix.env }}

Because composite actions cannot read the matrix context,
every consumer repo had to implement the same apply-to-$GITHUB_ENV boilerplate
in its own .github/workflows/custom/before-install/action.yml
(dm, duckdb-r, and rigraph each carry an identical copy today),
and a repo without that boilerplate silently drops the variables.

The claim that surfaced in igraph/rigraph —
"a before-install action is needed to pick up the environment variable" —
was therefore true for all consumer repos, not just rigraph.
It is an artifact of the template design, not of that repo.

CI logs confirm the failure mode for matrix values
referenced directly inside composite actions:
in cynkra/dm's "SQL Server with covr" job,
DM_TEST_SRC (set from ${{ matrix.config.test-src }} inside the composite action)
comes out empty, while the forwarded-input route (DM_VALIDATE=true) works.

Fix

• Apply the env field directly in the rcc-full job,
  before the custom before-install action runs,
  so the variables reach every subsequent step —
  including the custom before-install and after-install actions —
  in all repos, with no custom amendments needed.
• Drop the now-redundant env input forwarding.
• Escape backslash, double quote, and newline
  in the JSON emitted by versions-matrix/action.R,
  so multi-line env values (one KEY=VALUE per line)
  survive the trip through the matrix JSON.

Companion PRs

• ci: Sync R-CMD-check workflow; drop matrix-env plumbing from custom action igraph/rigraph#2798 — sync + drop the apply boilerplate
• ci: Restore database backend runs via the generic matrix env field dm#2486 — sync + drop boilerplate;
  also restores the six database-backend runs,
  which had been silently testing plain data frames
• ci: Sync R-CMD-check workflow; drop matrix-env plumbing from custom action duckdb/duckdb-r#2422 — sync + drop boilerplate
  (branch also pushed to krlmlr/duckdb-r)
• ci: Sync R-CMD-check workflow from cynkratemplate r-dbi/RMariaDB#547 — sync only;
  documents that its matrix.config.*-based client/server switches are inert
• ci: Sync R-CMD-check workflow from cynkratemplate r-dbi/RPostgres#592 — sync only

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https://claude.ai/code/session_0136C9bRMjBGkUrtLdkYNmAg