Rollup merge of #141904 - jieyouxu:test-float-parse, r=Mark-Simulacrum

test-float-parse: apply `cfg(not(bootstrap))`

Prior to stage 0 redesign, `test-float-parse` ran against in-tree std but now it runs against beta std. `f16::FromStr` is only present in in-tree std and not yet beta std, so apply `cfg(not(bootstrap))` gating to unbrick `./x check --stage=0`.

Fixes rust-lang/rust#141900.

`./x check --stage=0` in CI is intended for follow-up.

r? `@Mark-Simulacrum` (or bootstrap/libs)
cc `@tgross35`