Makefile: fix compilation on linux for Swift 6.

as specified in swiftlang/swift-package-manager@d1fdbbf
passing multiple archs is intended to allow compiling universal
binaries in macOS.
When compiling for Swift 6, passing these arguments in linux fails due
to the lack of xcode. For some reason this didn't prevent building prior
to Swift 6.

Makefile

@@ -6,8 +6,7 @@ UNAME_S := $(shell uname -s)
 
 ifeq ($(UNAME_S), Darwin)
 	BUILD_PATH_PREFIX := .build/apple/Products/Release
-endif
-ifeq ($(UNAME_S), Linux)
+else ifeq ($(UNAME_S), Linux)
 	BUILD_PATH_PREFIX := .build/release
 endif
 
@@ -25,7 +24,11 @@ install: build
 	cp -f $(BUILD_PATH) $(INSTALL_PATH)
 
 build:
+ifeq ($(UNAME_S), Darwin) 
 	swift build --disable-sandbox -c release --arch arm64 --arch x86_64
+else ifeq ($(UNAME_S), Linux) 
+	swift build --disable-sandbox -c release
+endif
 
 uninstall:
 	rm -f $(INSTALL_PATH)