correct rstan's dependency list and pin the package under test

Drop V8 and withr: rstan 2.32 replaced V8 with QuickJSR, and V8 failing to
install on an old toolchain would have failed the job for reasons that have
nothing to do with RcppParallel. The list now mirrors rstan's Imports plus
LinkingTo.

Install StanHeaders and rstan with dependencies = FALSE, and assert
afterwards that RcppParallel is still the .9000 build from this branch --
otherwise dependency resolution could quietly replace it with CRAN's, and
the run would report on the released package instead of this one.

Author: kevinushey

.github/workflows/rstan.yaml

@@ -69,12 +69,23 @@ jobs:
           ))'
 
       # binaries are fine for everything except rstan and StanHeaders, which are
-      # the packages that actually consume RcppParallel's flags
+      # the packages that actually consume RcppParallel's flags. this is
+      # rstan's Imports plus its LinkingTo, minus RcppParallel (installed above)
+      # and StanHeaders (built from source below)
       - name: install rstan's dependencies
-        run: Rscript -e 'install.packages(c("Rcpp","RcppEigen","BH","inline","loo","pkgbuild","QuickJSR","withr","V8","gridExtra","ggplot2"))'
+        run: |
+          Rscript -e 'install.packages(c("Rcpp","RcppEigen","BH","inline","gridExtra","loo","pkgbuild","QuickJSR","ggplot2"))'
 
+      # dependencies = FALSE so that resolving StanHeaders/rstan can't quietly
+      # pull CRAN's RcppParallel over the one we just built -- that would turn
+      # this into a test of the released package instead
       - name: install StanHeaders and rstan from source
-        run: Rscript -e 'install.packages(c("StanHeaders","rstan"), type = "source", INSTALL_opts = "--no-multiarch")'
+        run: |
+          Rscript -e 'install.packages(c("StanHeaders","rstan"), type = "source", dependencies = FALSE, INSTALL_opts = "--no-multiarch")'
+
+      - name: check RcppParallel is still the one we built
+        run: |
+          Rscript -e 'v <- as.character(packageVersion("RcppParallel")); writeLines(paste("RcppParallel", v)); if (!grepl("[.]9000$", v)) stop("RcppParallel was replaced by a released build; the rstan build did not test this branch")'
 
       - name: load rstan
         run: |