Use separate build directories for Rust libraries in dune

Currently these all use default/target, which seems to cause
difficult-to-debug race conditions in OSS CI.
Strangely, (locks /cargo) is supposed to prevent this given that locks
with an absolute path is documented by dune to be a global lock, but it
doesn't seem to work as intended.

Author: mszabo-wikia

hphp/hack/Makefile.dune

@@ -32,18 +32,13 @@ clean:
 define build_hack
 $(eval ext := $(if $(filter $(2),-debug),".bc",""))
 build-hack$(2):
-	rm -rf "$(DUNE_BUILD_DIR)/default/target/release"
-	dune build src/hh_server.$(1)
-	rm -rf "$(DUNE_BUILD_DIR)/default/target/release"
-	dune build src/hh_client.$(1)
-	rm -rf "$(DUNE_BUILD_DIR)/default/target/release"
-	dune build src/hh_single_type_check.$(1)
-	rm -rf "$(DUNE_BUILD_DIR)/default/target/release"
-	dune build src/hackfmt.$(1)
-	rm -rf "$(DUNE_BUILD_DIR)/default/target/release"
-	dune build src/hh_parse.$(1)
-	rm -rf "$(DUNE_BUILD_DIR)/default/target/release"
-	dune build src/generate_full_fidelity.$(1)
+	dune build \
+		src/hh_server.$(1) \
+		src/hh_client.$(1) \
+		src/hh_single_type_check.$(1) \
+		src/hackfmt.$(1) \
+		src/hh_parse.$(1) \
+		src/generate_full_fidelity.$(1)
 
 copy-hack$(2)-files: build-hack$(2)
 	mkdir -p "$(HACK_BIN_DIR)"