diff --git a/.github/versions-matrix.R b/.github/versions-matrix.R
index 33a7b24c0..1bc7dd517 100644
--- a/.github/versions-matrix.R
+++ b/.github/versions-matrix.R
@@ -1,14 +1,25 @@
 list(
+  # Backend runs: DM_TEST_SRC selects the database backend
+  # (see tests/testthat/helper-src.R), and the custom after-install action
+  # provisions the matching server. The variables travel through the generic
+  # "env" matrix field, which the rcc-full job applies to the environment of
+  # all steps. SKIP_UPDATE_SNAPSHOTS opts these entries out of the snapshot
+  # updater, which would otherwise run the test suite a second time against
+  # the backend and accept backend-specific snapshots.
   data.frame(
     os = "ubuntu-22.04",
     r = "release",
-    test_src = c(
-      "test-mssql",
-      "test-postgres",
-      "test-maria",
-      "test-mysql-maria",
-      "test-duckdb",
-      "test-sqlite"
+    env = paste0(
+      "DM_TEST_SRC=",
+      c(
+        "test-mssql",
+        "test-postgres",
+        "test-maria",
+        "test-mysql-maria",
+        "test-duckdb",
+        "test-sqlite"
+      ),
+      "\nSKIP_UPDATE_SNAPSHOTS=true"
     ),
     covr = "true",
     desc = c(
@@ -20,14 +31,14 @@ list(
       "SQLite with covr"
     )
   ),
-  # Instrumented validation run. The DM_VALIDATE flag travels through the
-  # generic "env" matrix field to the custom before/after-install actions,
-  # which uncomment code marked "# INSTRUMENT: validate" and opt out of the
-  # snapshot updater. covr then builds and tests the instrumented sources.
+  # Instrumented validation run. DM_VALIDATE makes the custom after-install
+  # action uncomment code marked "# INSTRUMENT: validate"; covr then builds
+  # and tests the instrumented sources. SKIP_UPDATE_SNAPSHOTS opts out of the
+  # snapshot updater, which would otherwise run the instrumented tests twice.
   data.frame(
     os = "ubuntu-22.04",
     r = "release",
-    env = "DM_VALIDATE=true",
+    env = "DM_VALIDATE=true\nSKIP_UPDATE_SNAPSHOTS=true",
     covr = "true",
     desc = "instrumented validation"
   )
diff --git a/.github/workflows/custom/before-install/action.yml b/.github/workflows/custom/before-install/action.yml
index 660a6a20f..cfb50641d 100644
--- a/.github/workflows/custom/before-install/action.yml
+++ b/.github/workflows/custom/before-install/action.yml
@@ -1,45 +1,19 @@
 name: 'Custom steps to run before R packages are installed'
 
-inputs:
-  env:
-    description: >
-      Additional environment variables (one KEY=VALUE per line) contributed by
-      the test matrix. Generic hook used by .github/versions-matrix.R to flag
-      special matrix entries (e.g. the instrumented-validation entry below).
-    required: false
-    default: ""
+# Environment variables from the "env" field of test matrix entries
+# (see .github/versions-matrix.R: DM_TEST_SRC, DM_VALIDATE,
+# SKIP_UPDATE_SNAPSHOTS) are applied by the rcc-full job in R-CMD-check.yaml
+# before this action runs, so they are already visible here
+# and in all later steps.
 
 runs:
   using: "composite"
   steps:
-    - name: Apply environment variables from the test matrix
-      if: inputs.env != ''
-      env:
-        MATRIX_ENV: ${{ inputs.env }}
-      run: |
-        printf '%s\n' "$MATRIX_ENV" | tee -a "$GITHUB_ENV"
-      shell: bash
-
-    # The instrumented-validation matrix entry sets DM_VALIDATE=true via the
-    # generic "env" field. Opt it out of the snapshot updater, which bypasses
-    # tests/testthat.R via testthat::test_local() and would otherwise run the
-    # instrumented tests twice.
-    - name: Opt out of the snapshot updater (instrumented validation)
-      if: env.DM_VALIDATE != ''
-      run: |
-        echo 'SKIP_UPDATE_SNAPSHOTS=true' | tee -a "$GITHUB_ENV"
-      shell: bash
-
     - name: Define _R_CHECK_PKG_SIZES_THRESHOLD_
       run: |
         echo '_R_CHECK_PKG_SIZES_THRESHOLD_=10' | tee -a $GITHUB_ENV
       shell: bash
 
-    - name: Define DM_TEST_SRC
-      run: |
-        echo "DM_TEST_SRC=${{ matrix.config.test-src }}" | tee -a $GITHUB_ENV
-      shell: bash
-
     - name: Clean up broken mysql apt
       # FIXME: Remove if package becomes unavailable
       if: runner.os == 'Linux'