Use cargo check -Zbuild-std instead of cargo miri setup

This should be more reliable (not building with `cfg(miri)` and more
accurately knowing whether a target supports std).

Known targets affected by this change:
- cygwin: should now build since it only breaks in `cfg(miri)`
- i686-pc-nto-qnx700: officially doesn't support std

closes #4

migrations/20250705120351_rename-miri-std.sql

@@ -0,0 +1,3 @@
+UPDATE finished_nightly SET mode = 'std' WHERE mode = 'miri-std';
+
+UPDATE build_info SET mode = 'std' WHERE mode = 'miri-std';