Set _R_CHECK_FORCE_SUGGESTS_=false in GHA workflow

XGR, haploR, and motifbreakR are optional Suggests that cannot be
installed on all CI platforms. All usages are guarded with
requireNamespace().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bschilder

.github/workflows/rworkflows.yml

@@ -18,6 +18,8 @@ jobs:
     runs-on: ${{ matrix.config.os }}
     name: ${{ matrix.config.os }} (${{ matrix.config.r }})
     container: ${{ matrix.config.cont }}
+    env:
+      _R_CHECK_FORCE_SUGGESTS_: false
     strategy:
       fail-fast: ${{ false }}
       matrix: