GH-47941: [R] Fix codegen.R error from dplyr pipe to base pipe change (#47985)

### Rationale for this change

Switching to base pipe means that we need to update some syntax in codegen.R otherwise it errors when run

### What changes are included in this PR?

Factor out code into own function so can use base pipe

### Are these changes tested?

No, though weird it didn't fail on CI - looking into this!

### Are there any user-facing changes?

Nope
* GitHub Issue: #47941

Authored-by: Nic Crane <[email protected]>
Signed-off-by: Nic Crane <[email protected]>

Author: thisisnic

r/configure.win

@@ -31,7 +31,7 @@ ARROW_R_DEV=`echo $ARROW_R_DEV | tr '[:upper:]' '[:lower:]'`
 ARROW_USE_PKG_CONFIG=`echo $ARROW_USE_PKG_CONFIG | tr '[:upper:]' '[:lower:]'`
 
 # generate code
-if [ "$ARROW_R_DEV" == "true" ]; then
+if [ "$ARROW_R_DEV" = "true" ] && [ -f "data-raw/codegen.R" ]; then
   echo "*** Generating code with data-raw/codegen.R"
   "${R_HOME}/bin${R_ARCH_BIN}/Rscript.exe" data-raw/codegen.R
 fi

r/data-raw/codegen.R

@@ -40,13 +40,15 @@ suppressPackageStartupMessages({
   library(vctrs)
 })
 
+cbind_unnested_functions <- function(data) {
+  vec_cbind(data, vec_rbind(!!!pull(data, functions)))
+}
+
 get_exported_functions <- function(decorations, export_tag) {
   out <- decorations |>
     filter(decoration %in% paste0(export_tag, "::export")) |>
     mutate(functions = map(context, decor:::parse_cpp_function)) |>
-    {
-      vec_cbind(., vec_rbind(!!!pull(., functions)))
-    } |>
+    cbind_unnested_functions() |>
     select(-functions) |>
     mutate(decoration = sub("::export", "", decoration))
   message(glue(