Add needed --experimental-options for chunky C-ext benchmarks

Author: eregon

ci.jsonnet

@@ -473,7 +473,7 @@ local part_definitions = {
 
     cext_chunky: {
       environment+: {
-        TRUFFLERUBYOPT: "--cexts-log-load=true",
+        TRUFFLERUBYOPT: "--experimental-options --cexts-log-load",
         USE_CEXTS: "true",
       },
       setup+:

doc/contributor/cexts.md

@@ -40,7 +40,7 @@ jt cextc bench/chunky_png/oily_png
 Then follow the instructions for benchmarking above, and then try:
 
 ```bash
-$ USE_CEXTS=true TRUFFLERUBYOPT=--cexts-log-load jt benchmark bench/chunky_png/chunky-color-r.rb --simple
+$ USE_CEXTS=true TRUFFLERUBYOPT="--experimental-options --cexts-log-load" jt benchmark bench/chunky_png/chunky-color-r.rb --simple
 ```
 
 These benchmarks have Ruby fallbacks, so we should carefully check that the