fix multi-line Rscript -e in the rstan workflow

On Windows, 'Rscript -e' receives only the first line of a multi-line
argument, so the reporting step failed to parse what it did get and
aborted the run before rstan was ever built. Move the code into a
script file, as the other steps already do.

Author: kevinushey

.github/scripts/report-tbb-config.R

@@ -0,0 +1,14 @@
+# Report how the installed RcppParallel resolved TBB, so that a downstream
+# build failure can be read against the configuration that produced it.
+#
+# This lives in a file rather than inline in the workflow: 'Rscript -e' only
+# receives the first line of a multi-line argument on Windows, so the rest of
+# the expression is silently dropped and R fails to parse what it did get.
+
+writeLines(c(
+   sprintf("TBB_ENABLED : %s", RcppParallel:::TBB_ENABLED),
+   sprintf("TBB_LIB     : '%s'", RcppParallel:::TBB_LIB),
+   sprintf("TBB_INC     : '%s'", RcppParallel:::TBB_INC),
+   sprintf("CxxFlags()  : %s", RcppParallel:::tbbCxxFlags()),
+   sprintf("LdFlags()   : %s", RcppParallel:::tbbLdFlags())
+))

.github/workflows/rstan.yaml

@@ -60,13 +60,7 @@ jobs:
         run: R CMD INSTALL --preclean .
 
       - name: report how RcppParallel resolved TBB
-        run: |
-          Rscript -e 'writeLines(c(
-            paste("TBB_ENABLED", RcppParallel:::TBB_ENABLED),
-            paste("TBB_LIB   ", RcppParallel:::TBB_LIB),
-            paste("CxxFlags  ", RcppParallel:::tbbCxxFlags()),
-            paste("LdFlags   ", RcppParallel:::tbbLdFlags())
-          ))'
+        run: Rscript .github/scripts/report-tbb-config.R
 
       # binaries are fine for everything except rstan and StanHeaders, which are
       # the packages that actually consume RcppParallel's flags. this is