d/rules: Only remove Cargo.lock if it exists

This allows to invoke the build target twice in a row

debian/rules

@@ -9,7 +9,7 @@ export DEB_BUILD_MAINT_OPTIONS = hardening=+all
 # The Debian version of linked-hash-map doesn't provide any hash,
 # causing Cargo to refuse to build with a crates.io copy
 build-arch:
-	rm Cargo.lock
+	[ ! -f Cargo.lock ] || rm Cargo.lock
 	dh $@ --builddirectory=_build --buildsystem=meson
 
 override_dh_autoreconf: