Only run integration tests on Linux x86-64, macOS, and Windows

There's no point running these integration tests with other non-x86_64 archs
on Linux. I'm not sure exactly what Cross is doing under the hood, but the
tests all seem to end up using the x86-64 arch, not the target arch.

tests/ubi.rs

@@ -43,6 +43,15 @@ impl PreservableTempdir {
     }
 }
 
+// There's no point running these integration tests with other non-x86_64
+// archs on Linux. I'm not sure exactly what Cross is doing under the hood,
+// but the tests all seem to end up using the x86-64 arch, not the target
+// arch.
+#[cfg(any(
+    all(target_os = "linux", target_arch = "x86_64"),
+    target_os = "macos",
+    target_os = "windows"
+))]
 #[test]
 fn tests() -> Result<()> {
     let cargo = make_exe_pathbuf(&["cargo"]);