extendr: 32-bit Windows cannot handle Rfloat::na()

c.f. https://github.com/extendr/extendr/pull/318#issuecomment-964282455

failures:

---- src\scalar\rfloat.rs - scalar::rfloat::Rfloat (line 35) stdout ----
Test executable failed (exit code 101).

stderr:
thread 'main' panicked at 'assertion failed: (<Rfloat>::na()).is_na()', src\scalar\rfloat.rs:6:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


---- src\wrapper\doubles.rs - wrapper::doubles::Doubles::elt (line 25) stdout ----
Test executable failed (exit code 101).

stderr:
thread 'main' panicked at 'assertion failed: vec.elt(10).is_na()', src\wrapper\doubles.rs:9:4
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

For this reason, one ALTREP-related test is disabled. Let’s not forget to re-enable this as well.

https://github.com/extendr/extendr/pull/318/commits/473ac0597683aee9e2c2aae9497f8f1d794ceff3