Only build swiftlint target in Makefile (realm#4327)

Otherwise this tries to build the SwiftLintTestHelpers module in release
mode, which fails.

Also, to build the actual executable CLI, we can't just
`swift build --target=swiftlint` because that doesn't produce a CLI,
just the object file for the binary. So we have to `swift run`.

Makefile

@@ -69,10 +69,10 @@ clean_xcode:
 	$(BUILD_TOOL) $(XCODEFLAGS) -configuration Test clean
 
 build_x86_64:
-	swift build $(SWIFT_BUILD_FLAGS) --arch x86_64
+	swift run $(SWIFT_BUILD_FLAGS) --arch x86_64 swiftlint version
 
 build_arm64:
-	swift build $(SWIFT_BUILD_FLAGS) --arch arm64
+	swift run $(SWIFT_BUILD_FLAGS) --arch arm64 swiftlint version
 
 build: clean build_x86_64 build_arm64
 	# Need to build for each arch independently to work around https://bugs.swift.org/browse/SR-15802