chore: add experiment toggle for terraform workspace caching (#20559)

Experiments passed to provisioners to determine behavior. This adds
`--experiments` flag to provisioner daemons. Prior to this, provisioners
had no method to turn on/off experiments.

enterprise/cli/testdata/coder_provisioner_start_--help.golden

@@ -6,6 +6,11 @@ USAGE:
   Run a provisioner daemon
 
 OPTIONS:
+      --experiments string-array, $CODER_EXPERIMENTS
+          Enable one or more experiments. These are not ready for production.
+          Separate multiple experiments with commas, or enter '*' to opt-in to
+          all available experiments.
+
   -O, --org string, $CODER_ORGANIZATION
           Select which organization (uuid or name) to use.