Run standard library unit tests without optimizations in `nopt` CI jobs
This was discussed in #73288 as a way to catch similar issues in the future. This builds an unoptimized standard library with the bootstrap compiler and runs the unit tests. This takes about 2 minutes on my laptop.
I confirmed that this method works locally, although there may be a better way of implementing it. It would be better to use the stage 2 compiler instead of the bootstrap one.
Notably, there are currently four `libstd` unit tests that fail in debug mode on `i686-unkown-linux-gnu` (a tier one target):
```
failures:
f32::tests::test_float_bits_conv
f32::tests::test_total_cmp
f64::tests::test_float_bits_conv
f64::tests::test_total_cmp
```
These are the tests that prompted #73288 as well as the ones added in #72568, which is currently broken due to #73328.
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