Use a local switch in playground Makefile (#1469)

---------

Co-authored-by: Sabine Schmaltz <[email protected]>

playground/Makefile

@@ -1,17 +1,19 @@
 COMPILER=5.0.0
-SWITCH=playground
 
 .PHONY: create_switch
-create_switch: ## Create switch and pinned opam repo
-	opam switch create $(SWITCH) $(COMPILER) --no-install
+create_switch: ## Create switch
+	opam switch create . $(COMPILER) --no-install
 
 .PHONY: deps
 deps: create_switch ## Install development dependencies
-	opam install . --deps-only --switch=$(SWITCH)
+	opam install . --deps-only -y
+
+.PHONY: switch
+switch: deps ## Create an opam switch and install development dependencies
 
 .PHONY: build
 build:
-	opam exec --switch=$(SWITCH) -- dune build --root .
+	opam exec -- dune build --root .
 
 .PHONY: fmt
 fmt: ## Format the codebase with ocamlformat